A method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, and its application.

Abstract

This invention relates to the field of lithium-ion battery technology, and proposes a method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, and its application. The method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, includes the following steps: dissolving the ferrous sulfate byproduct of titanium dioxide production, adding a purifying agent for purification, filtering, and obtaining a purified ferrous sulfate solution; using the purified ferrous sulfate solution as an electrolyte, Fe... 2+ Partially oxidized to Fe 3+ Phosphoric acid is added to the obtained mixed-valence iron solution to adjust the pH, and the mixture is aged to form an iron phosphate precursor slurry. The iron phosphate precursor slurry is mixed with a lithium hydroxide ethanolamine-water mixed solution and reacted in a microwave reactor to obtain a lithium iron phosphate precursor suspension. The lithium iron phosphate precursor is crystallized, and carbon source and silicon source vapors are simultaneously introduced for gas-phase surface modification to obtain lithium iron phosphate material. This invention improves the purity of raw materials, improves the powder structure, and enhances long-term cycling stability.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, specifically to a method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide, and its application. Background Technology

[0002] Lithium iron phosphate (LiFePO4), as an important cathode material for lithium-ion batteries, has been widely used in electric vehicles and energy storage due to its excellent safety, long cycle life, and environmental friendliness. With the rapid development of industrial technology, higher demands are being placed on the accuracy and comparability of lithium iron phosphate material performance evaluation. In recent years, research on the preparation of lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, has received widespread attention. Numerous researchers and companies have conducted extensive research in this field, achieving a series of important results. Currently, common methods for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, mainly include solid-phase methods, liquid-phase methods (such as hydrothermal methods and co-precipitation methods), and sol-gel methods.

[0003] Ferrous sulfate, a byproduct of titanium dioxide production, typically contains various impurities, such as heavy metal ions (e.g., copper, zinc, manganese), organic impurities, and colloidal impurities. These impurities are easily introduced into the lithium iron phosphate (LFP) crystal lattice during subsequent preparation processes, forming lattice defects. These defects disrupt the crystal structure of LFP, affecting lithium ion insertion and extraction, thereby reducing the material's electrochemical performance, such as specific capacity, cycle stability, and rate performance. The formation of the iron phosphate precursor is a crucial step in the preparation of LFP from ferrous sulfate, a byproduct of titanium dioxide production. Existing methods often encounter complex phase transition issues during precursor formation. Traditional preparation methods result in low synthesis rates. Although some progress has been made in the preparation of LFP from ferrous sulfate, a byproduct of titanium dioxide production, problems remain regarding raw material purity, precursor formation process, synthesis efficiency, and electrochemical performance. Therefore, this invention proposes a method for preparing LFP from ferrous sulfate, a byproduct of titanium dioxide production, and its application. Summary of the Invention

[0004] This invention proposes a method and application for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production. This method improves the purity of raw materials and avoids the introduction of lattice defects by impurities; it improves the formation process of the iron phosphate precursor and avoids complex phase transitions; it enhances the synthesis efficiency and product morphology control; it improves the powder structure, enhances electrochemical performance, reduces interfacial side reactions, and improves long-term cycling stability.

[0005] The technical solution of the present invention is as follows:

[0006] This invention proposes a method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, comprising the following steps:

[0007] (1) Purification treatment: After dissolving the titanium dioxide byproduct ferrous sulfate, add disodium ethylenediaminetetraacetate, citric acid, ascorbic acid and polyacrylamide as a composite purifying agent for purification treatment. After the reaction, filter to obtain a purified ferrous sulfate solution.

[0008] (2) Electrochemical control: The purified ferrous sulfate solution is used as the electrolyte, and Fe is controlled by electrochemical methods. 2+ Partially oxidized to Fe 3+ Phosphoric acid was then added to the resulting mixed valence iron solution, and the pH was adjusted with ammonia. After aging, an iron phosphate precursor slurry was formed.

[0009] (3) Microwave synthesis: The iron phosphate precursor slurry and the lithium hydroxide ethanolamine-water mixed solution are mixed by a microfluidic mixer and then reacted in a microwave reactor to obtain a lithium iron phosphate precursor suspension;

[0010] (4) Drying and crystallization: The lithium iron phosphate precursor is separated, dried by supercritical CO2, and then crystallized in a fluidized bed reactor. Simultaneously, carbon source and silicon source vapor are introduced for gas-phase surface modification to obtain carbon-silicon dual-phase coated lithium iron phosphate material.

[0011] As a further technical solution, in step (1), the composite purifying agent comprises 10% by mass of disodium ethylenediaminetetraacetate solution, 15% by mass of citric acid solution, 5% by mass of ascorbic acid solution and 0.5% by mass of polyacrylamide solution.

[0012] As a further technical solution, the purification reaction conditions are: pH 3.5-4.0, temperature 55-65℃, and time 1.5-2.5h.

[0013] As a further technical solution, in step (2), the working electrode of the three-electrode system is a titanium-based IrO2-Ta2O5 coated anode, with a controlled potential of +0.7-0.8V vs. Ag / AgCl, and the Fe... 2+ The oxidation ratio is 20%-40%.

[0014] As a further technical solution, in step (2), the amount of phosphoric acid added is 1.01-1.05 times the total molar amount of iron in the solution, the pH of the mixed solution is adjusted to 2.5-3.0 using ammonia water, and aged at 50-70℃ for 0.5-1.5h.

[0015] As a further technical solution, in step (3), the concentration of lithium hydroxide in the ethanolamine-water mixed solution of lithium hydroxide is 1.8-2.2 mol / L, the volume ratio of ethanolamine to water is 1:3-1:5, and the molar ratio of lithium source to iron (Li:Fe) is 1.01-1.05:1.

[0016] As a further technical solution, the conditions for the microwave reaction in step (3) are: power 500-1000W, reaction temperature 115-135℃, pressure 0.4-0.8MPa, and residence time 10-15min.

[0017] As a further technical solution, the conditions for supercritical CO2 drying in step (4) are: pressure 8-12MPa, temperature 35-45℃, and time 3-5h.

[0018] As a further technical solution, the crystallization conditions in step (4) are: nitrogen atmosphere, temperature 430-470℃, time 1.5-2.5h; the gas phase surface modification includes simultaneously introducing ethylene as a carbon source and tetraethoxysilane vapor as a silicon source.

[0019] The present invention also proposes the application of lithium iron phosphate prepared by the method described above using ferrous sulfate, a byproduct of titanium dioxide, in lithium-ion batteries.

[0020] The working principle and beneficial effects of this invention are as follows:

[0021] This invention utilizes a composite purifying agent composed of disodium ethylenediaminetetraacetate, citric acid, ascorbic acid, and polyacrylamide. Disodium ethylenediaminetetraacetate can complex some heavy metal ions, while citric acid can synergistically complex with various ions, further enhancing its ability to complex impurities. Ascorbic acid has antioxidant protective effects, preventing Fe... 2+ Oxidation ensures the stability of the iron valence state in the solution; polyacrylamide can flocculate colloidal impurities, making the impurity particles aggregate for easy separation; these four purifying agents work together to comprehensively and efficiently remove impurity elements from the raw materials through complexation, anti-oxidation, flocculation and other aspects, avoiding the introduction of impurities as lattice defects in subsequent steps, thus laying a good raw material foundation for the preparation of high-performance lithium iron phosphate materials.

[0022] This invention uses an electrochemical method to control Fe 2+ Partially oxidized to Fe 3+ By using a three-electrode system with a titanium-based IrO2-Ta2O5 coated anode, the potential was controlled at +0.7-0.8V vs. Ag / AgCl, allowing for precise regulation of Fe. 2+The oxidation ratio is 20-40%; this mixed-valence iron solution, after adding phosphoric acid and adjusting the pH, is aged to form an iron phosphate precursor slurry, with an appropriate amount of Fe. 3+ It contributes to the initial nucleation site and can regulate crystal growth kinetics, avoiding the complex phase transitions that occur in single-valence iron during subsequent lithiation, reduction, and crystallization processes. This is conducive to the formation of well-crystallized and structurally stable iron phosphate precursors, providing key precursor conditions for the final preparation of high-performance lithium iron phosphate materials.

[0023] This invention employs a microwave-assisted continuous synthesis process. The iron phosphate precursor slurry is mixed with a lithium hydroxide ethanolamine-water solution via a microfluidic mixer before being fed into a microwave reactor. Microwave heating is highly efficient and uniform, enabling the reaction system to reach the required temperature quickly, rapidly forming a well-crystallized, fine-particle, and uniform precursor. This heating method overcomes the shortcomings of traditional hydrothermal methods, offering significant advantages in synthesis efficiency and product morphology control, and providing a high-quality intermediate product for the preparation of high-performance lithium iron phosphate materials.

[0024] This invention employs supercritical CO2 drying, which effectively eliminates capillary forces, resulting in a loose, porous powder structure free of hard agglomerates. This maintains the material's high specific surface area, good particle dispersibility, and rapid ion transport capabilities. Simultaneously, during crystallization, carbon source (ethylene) and silicon source (tetraethoxysilane vapor) vapors are introduced for gas-phase surface modification. Ethylene decomposition forms a carbon coating layer, successfully constructing a highly efficient electronic conductivity network and solving the bottleneck problem of the intrinsically low electronic conductivity of LiFePO4 materials. The amorphous SiO2 thin layer generated from the pyrolysis of tetraethoxysilane further stabilizes the electrode / electrolyte interface, reducing interfacial side reactions during long-term cycling, and decreasing interfacial impedance growth and active lithium loss. Supercritical CO2 drying and gas-phase surface modification work synergistically to improve the electrochemical performance of the material, particularly its long-term cycling stability, from both particle structure and surface properties perspectives. Detailed Implementation

[0025] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. It should be noted that the cellulase and pectinase used in the present invention were purchased from Shandong Longket Enzyme Preparation Co., Ltd.

[0026] Example 1

[0027] This embodiment provides a method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, comprising the following steps:

[0028] (1) Purification treatment of ferrous sulfate: Take 100 kg of titanium dioxide by-product ferrous sulfate heptahydrate, add 300 L of deionized water to dissolve it and prepare a uniform ferrous sulfate solution: prepare 15 L of 10% aqueous solution of disodium ethylenediaminetetraacetate; prepare 13.3 L of 15% aqueous solution of citric acid; prepare 10 L of 5% aqueous solution of ascorbic acid; prepare 2 L of 0.5% aqueous solution of polyacrylamide; slowly add the above prepared solutions to the ferrous sulfate solution in sequence, and react for 2 h at a stirring speed of 200 r / min under the conditions of 60℃ and pH 3.8. After the reaction is completed, filter the solution with a filter membrane with a pore size of 0.45 μm to obtain a purified ferrous sulfate solution. The purity of ferrous sulfate in the solution is detected by potassium dichromate titration to ensure that the purity is ≥99.5%.

[0029] (2) Electrochemical control: 10 L of purified ferrous sulfate solution with a concentration of 1.5 mol / L was used as the electrolyte; a three-electrode system was adopted: titanium-based IrO2-Ta2O5 coated anode, graphite cathode, and Ag / AgCl reference electrode; the potential was controlled at +0.75 V vs. Ag / AgCl, and samples were taken every 10 min. The Fe concentration in the solution was determined by o-phenanthroline spectrophotometry. 2+ and Fe 3+ The content of Fe, electrolyzed to 30% 2+ Oxidized to Fe 3+ A mixed-valence iron solution was obtained, with a total iron concentration of approximately 1.5 mol / L. 1.05 L of 85% phosphoric acid aqueous solution was slowly added at 200 rpm, and the pH of the mixed solution was adjusted to 2.8 using ammonia solution. The solution was aged at 60 °C for 1 h to form a uniform iron phosphate precursor slurry.

[0030] (3) Microwave synthesis: Prepare lithium source solution: lithium hydroxide concentration 2.0 mol / L, solvent is ethanolamine-water mixed solvent (volume ratio 1:4); mix 10 L of iron phosphate precursor slurry with 3.82 L of lithium source solution through a microfluidic mixer and enter microwave reactor; reaction conditions: microwave power 800 W, temperature 125 ℃, pressure 0.6 MPa, residence time 12 min, to obtain lithium iron phosphate precursor suspension;

[0031] (4) Drying and crystallization: After centrifugation, the precursor is dried by supercritical CO2: pressure 10MPa, temperature 40℃, time 4h; after drying, the material is placed in a fluidized bed reactor and crystallized at 450℃ for 2h under nitrogen atmosphere. At the same time, ethylene (200mL / min) and tetraethoxysilane vapor are introduced for surface modification to obtain the final product.

[0032] Example 2

[0033] This embodiment provides a method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, comprising the following steps:

[0034] (1) Purification treatment of ferrous sulfate: Take 100 kg of titanium dioxide by-product ferrous sulfate heptahydrate, add 300 L of deionized water to dissolve it and prepare a uniform ferrous sulfate solution: prepare 15 L of 10% aqueous solution of disodium ethylenediaminetetraacetate; prepare 13.3 L of 15% aqueous solution of citric acid; prepare 10 L of 5% aqueous solution of ascorbic acid; prepare 2 L of 0.5% aqueous solution of polyacrylamide; slowly add the above prepared solutions to the ferrous sulfate solution in sequence, and react at 55℃ and pH 3.5 with a stirring speed of 200 r / min for 1.5 h. After the reaction is completed, filter the solution with a filter membrane with a pore size of 0.45 μm to obtain a purified ferrous sulfate solution. The purity of ferrous sulfate in the solution is detected by potassium dichromate titration to ensure that the purity is ≥99.5%.

[0035] (2) Electrochemical control: 10 L of purified ferrous sulfate solution with a concentration of 1.5 mol / L was used as the electrolyte; a three-electrode system was adopted: titanium-based IrO2-Ta2O5 coated anode, graphite cathode, and Ag / AgCl reference electrode; the potential was controlled at +0.7 V vs. Ag / AgCl, and samples were taken every 10 min. The Fe concentration in the solution was determined by o-phenanthroline spectrophotometry. 2+ and Fe 3+ The content of Fe, electrolyzed to 20% 2+ Oxidized to Fe 3+ A mixed-valence iron solution was obtained with a total iron concentration of approximately 1.5 mol / L. 1.01 L of 85% phosphoric acid aqueous solution was slowly added at 200 rpm, and the pH of the mixed solution was adjusted to 2.5 using ammonia solution. The solution was aged at 50 °C for 0.5 h to form a uniform iron phosphate precursor slurry.

[0036] (3) Microwave synthesis: Prepare lithium source solution: lithium hydroxide concentration 1.8 mol / L, solvent is ethanolamine-water mixed solvent (volume ratio 1:3); mix 10L of iron phosphate precursor slurry with 4.17 L of lithium source solution through a microfluidic mixer and enter microwave reactor; reaction conditions: microwave power 500W, temperature 115℃, pressure 0.4MPa, residence time 10min to obtain lithium iron phosphate precursor suspension;

[0037] (4) Drying and crystallization: After centrifugation, the precursor is dried by supercritical CO2: pressure 8MPa, temperature 35℃, time 3h; after drying, the material is placed in a fluidized bed reactor and crystallized at 430℃ for 1.5h under nitrogen atmosphere. At the same time, ethylene (200mL / min) and tetraethoxysilane vapor are introduced for surface modification to obtain the final product.

[0038] Example 3

[0039] This embodiment provides a method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, comprising the following steps:

[0040] (1) Purification treatment of ferrous sulfate: Take 100 kg of titanium dioxide by-product ferrous sulfate heptahydrate, add 300 L of deionized water to dissolve it and prepare a uniform ferrous sulfate solution: prepare 15 L of 10% (w / w) aqueous solution of disodium ethylenediaminetetraacetate; prepare 13.3 L of 15% (w / w) aqueous solution of citric acid; prepare 10 L of 5% (w / w) aqueous solution of ascorbic acid; prepare 2 L of 0.5% (w / w) aqueous solution of polyacrylamide; slowly add the above prepared solutions to the ferrous sulfate solution in sequence, and react at 200 r / min for 2.5 h under the conditions of 65℃ and pH 4.0. After the reaction is completed, filter the solution with a filter membrane with a pore size of 0.45 μm to obtain a purified ferrous sulfate solution. The purity of ferrous sulfate in the solution is detected by potassium dichromate titration to ensure that the purity is ≥99.5%.

[0041] (2) Electrochemical control: 10 L of purified ferrous sulfate solution with a concentration of 1.5 mol / L was used as the electrolyte; a three-electrode system was adopted: titanium-based IrO2-Ta2O5 coated anode, graphite cathode, and Ag / AgCl reference electrode; the potential was controlled at +0.8 V vs. Ag / AgCl, and samples were taken every 10 min. The Fe concentration in the solution was determined by o-phenanthroline spectrophotometry. 2+ and Fe 3+ The content of Fe, electrolyzed to 40% 2+ Oxidized to Fe 3+ A mixed-valence iron solution was obtained with a total iron concentration of approximately 1.5 mol / L. 1.05 L of 85% phosphoric acid aqueous solution was slowly added at 200 rpm, and the pH of the mixed solution was adjusted to 3.0 using ammonia aqueous solution. The solution was aged at 70 °C for 1.5 h to form a uniform iron phosphate precursor slurry.

[0042] (3) Microwave synthesis: Prepare lithium source solution: lithium hydroxide concentration 2.2 mol / L, solvent is ethanolamine-water mixed solvent (volume ratio 1:5); mix 10L of iron phosphate precursor slurry with 3.48 L of lithium source solution through a microfluidic mixer and enter microwave reactor; reaction conditions: microwave power 1000W, temperature 135℃, pressure 0.8MPa, residence time 15min to obtain lithium iron phosphate precursor suspension;

[0043] (4) Drying and crystallization: After centrifugation, the precursor is dried by supercritical CO2: pressure 12MPa, temperature 45℃, time 5h; after drying, the material is placed in a fluidized bed reactor and crystallized at 470℃ for 2.5h under nitrogen atmosphere. At the same time, ethylene (200mL / min) and tetraethoxysilane vapor are introduced for surface modification to obtain the final product.

[0044] Comparative Example 1

[0045] This comparative example provides a method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production. Following step (1) of Example 1, a ferrous sulfate solution is obtained through purification, but without electrochemical control. 10 L of the purified ferrous sulfate solution is taken, and 2.5 L of 30% hydrogen peroxide solution is added. The solution is heated to 80 °C for 1 h to oxidize the Fe... 2+ Completely oxidized to Fe 3+ Ammonium phosphate solution (phosphate to iron molar ratio 1.05:1) was added, and the pH was adjusted to 2.5 with ammonia. The mixture was aged at 90℃ for 2 hours to form iron phosphate precipitate. The precipitate was separated by centrifugation and washed three times with deionized water. The iron phosphate precursor was mixed with lithium carbonate at a Li:Fe ratio of 1.05:1, ball-milled for 4 hours, and then sintered at 750℃ for 10 hours under an argon atmosphere to obtain lithium iron phosphate material.

[0046] Comparative Example 2

[0047] This comparative example provides a method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide. In step (1), only disodium ethylenediaminetetraacetate is used as a purifying agent (the amount is the same as the total complexing agent mass in Example 1), and citric acid, ascorbic acid, and polyacrylamide are not used. Other steps are the same as in Example 1.

[0048] Comparative Example 3

[0049] This comparative example provides a method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide. Following steps (1)-(2) of Example 1, after obtaining the iron phosphate precursor slurry, a lithium hydroxide solution (Li:Fe=1.05:1) is added and transferred to a high-pressure reactor. The mixture is then hydrothermally reacted at 180°C for 12 hours. The product is centrifuged, dried at 80°C, and sintered at 650°C for 8 hours under an argon atmosphere to obtain lithium iron phosphate material. The rest is the same as in Example 1.

[0050] Comparative Example 4

[0051] This comparative example provides a method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production. In step (2), the potential is controlled at +1.2V vs. Ag / AgCl, and electrolysis is performed until 95% Fe is obtained. 2+ Oxidized to Fe 3+ The other steps are the same as in Example 1.

[0052] Comparative Example 5

[0053] This comparative example provides a method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production. In step (2), the potential is controlled at +0.5V vs. Ag / AgCl, and electrolysis is performed until only 5% Fe remains. 2+ Oxidized to Fe 3+ The other steps are the same as in Example 1.

[0054] Comparative Example 6

[0055] This comparative example provides a method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide. In step (4), supercritical CO2 drying is not used; instead, vacuum drying at 80°C for 12 hours is performed. Other conditions are the same as in Example 1.

[0056] Comparative Example 7

[0057] This comparative example provides a method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide. In step (4), ethylene and tetraethoxysilane vapors are not introduced; crystallization is carried out only under a nitrogen atmosphere. Other conditions are the same as in Example 1.

[0058] Comparative Example 8

[0059] This comparative example provides a method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production. In step (4), only ethylene is introduced as a carbon source, and tetraethoxysilane vapor is not introduced. Other conditions are the same as in Example 1.

[0060] Experimental Example 1: The lithium iron phosphate materials obtained in Examples 1-3 and Comparative Examples 1-8 were prepared into electrodes and tested according to the following standard method. Lithium iron phosphate positive electrode material, PVDF, and carbon black were weighed in a mass ratio of 90:6:4. Then, an appropriate amount of NMP solvent was added to disperse them, and they were uniformly coated on aluminum foil. They were dried in an oven at 100°C for 8 hours, rolled, and cut into positive electrode sheets. In a vacuum glove box, lithium metal sheets were used as negative electrodes, PE / PP composite membranes were used as separators, LiPF6 was used as the electrolyte solute with a concentration of 1 mol / L, and ethylene carbonate (EC) and dimethyl carbonate (DMC) were used as electrolyte solvents with a volume ratio of 1:1. They were assembled into CR2430 coin cells.

[0061] The test results are shown in Table 1 below:

[0062] Table 1

[0063]

[0064] Based on the above data, it can be seen that the LiFePO4 material prepared by Comparative Example 1 using the traditional oxidation precipitation and high-temperature solid-state sintering method exhibits performance inferior to that of the embodiments of this invention. The 0.1C specific capacity is significantly lower, high-rate performance degradation is severe, and cycle stability is poor. Forced oxidation to Fe using hydrogen peroxide is also ineffective. 3+ Subsequently, carbothermic reduction and crystallization are carried out through a high-temperature, long-duration solid-state reaction, a process that is lengthy and uncontrollable. This results in the following inherent defects in the material: large and uneven particles, which are not conducive to rapid lithium-ion diffusion; low product purity, which easily leads to the formation of impurity phases; and insufficient construction of the internal conductive network. This demonstrates that the route of this invention, which combines precise electrochemical oxidation, liquid-phase microwave synthesis, and in-situ carbon coating, is significantly superior to traditional processes in improving the electrochemical activity and structural stability of the material.

[0065] Comparative Example 2 simplified the purification process, leading to a systematic decline in material performance. Compared to Example 1, capacity, rate performance, and cycle retention all decreased. For example, the 0.1C capacity decreased from 162.5 mAh / g to 152.0 mAh / g, and the 10C retention decreased from 85.3% to 68%. Using only disodium ethylenediaminetetraacetate as the purification agent, while capable of complexing some heavy metal ions, has a limited function. It lacks the synergistic complexation of multiple ions by citric acid and the complexation of Fe by ascorbic acid. 2+ The lack of antioxidant protection and the absence of polyacrylamide's flocculation effect on colloidal impurities resulted in incomplete removal of impurity elements from the raw materials. These impurities were introduced as lattice defects in subsequent steps, deteriorating the material's ion / electron transport properties and accelerating structural degradation during cycling.

[0066] Comparative Example 3, which used a conventional hydrothermal method instead of microwave synthesis, showed a significant decrease in material performance. The material capacity (150.5 mAh / g) was lower than that of the microwave-synthesized example. In particular, the high-rate performance (65% 10C retention) was far inferior to Example 1. The traditional hydrothermal method has a long reaction time and a large temperature gradient, resulting in uneven nucleation and growth, making it difficult to obtain ideal particle morphology with fine size and uniform distribution. Subsequent high-temperature calcination is still required, which is energy-intensive and easily causes particle agglomeration and growth. This leads to poor controllability of particle size and morphology, and long and non-uniform ion diffusion paths, thus affecting the kinetic performance at high rates. The microwave-assisted continuous synthesis process of this invention, with its efficient and uniform heating characteristics, can rapidly form precursors with good crystallinity and fine and uniform particles, and has outstanding advantages in synthesis efficiency and product morphology control.

[0067] Fe in Comparative Example 4 3+ When the Fe ratio is too high, the prepared material exhibits the worst electrochemical performance. All performance indicators are at their lowest levels, especially the 10C rate retention rate of only 52% and the cycle retention rate of only 84.1%. 2+ Over-oxidized to Fe 3+Subsequently, the formed phosphate precursor is closer to pure FePO4 than Fe3(PO4)2·8H2O or a mixture thereof. This precursor needs to undergo more complex phase transitions in the subsequent microwave synthesis process, which can easily lead to an increase in lattice defects, insufficient crystallinity, and unclear two-phase interfaces in the final LiFePO4, seriously impairing the intrinsic electrochemical activity and structural stability of the material.

[0068] Fe in Comparative Example 5 3+ A low proportion also leads to poor material performance, but the failure mechanism differs from that of Comparative Example 4. The performance degradation is slightly less severe than in Comparative Example 4, but still significantly worse than in the examples. Capacity retention is 86.5%, and 10C retention is 55%. Fe 3+ A low proportion means that the precursor is mainly Fe. 2+ Phosphate. In the subsequent microwave synthesis process, a suitable amount of Fe was lacking. 3+ The contribution to the initial nucleation site and its regulatory effect on its growth kinetics. This may lead to excessively rapid crystal growth, irregular morphology, and even partial Fe... 2+ The iron oxides were not effectively fixed and may form other iron oxide impurities during subsequent high-temperature processing. This crystal structure and composition affect the overall electrochemical performance of the material. This further verifies the presence of Fe. 2+ / Fe 3+ Precise control of mixed valence states is key to obtaining high-performance precursors and final products. Fe 3+ The introduction of [a specific ingredient] played a crucial role in optimizing the crystallization process and the final crystal quality.

[0069] Comparative Example 6 used conventional vacuum drying instead of supercritical CO2 drying, resulting in a decrease in the material's tap density and some electrochemical properties. Although the capacity and cycle retention rate did not decrease significantly, the high-rate performance (10C retention rate of 73.5%) was significantly lower than in Example 1. During conventional drying, the capillary forces generated by solvent (water / alcohol) evaporation cause hard agglomeration of nano / submicron-sized primary particles. These hard agglomerates are difficult to break down in subsequent electrode fabrication, leading to a reduction in the effective contact area between particles and an increase in the interfacial impedance for electron and ion transport; the diffusion path of lithium ions within the agglomerates becomes longer. These factors collectively weaken the material's ability to discharge at high rates. Supercritical CO2 drying can effectively eliminate capillary forces, obtaining a loose, porous powder structure free of hard agglomerates, which plays an important role in maintaining the material's high specific surface area, good particle dispersibility, and rapid ion transport capability.

[0070] Comparative Example 7, without any surface modification, exhibits complete material performance failure. Its 0.1C capacity is extremely low, decaying to 75% after only 200 cycles, and effective rate performance testing is impossible. This indicates severe battery polarization, rendering it unusable. LiFePO4 material itself has extremely low electronic conductivity. Without any carbon coating modification, the particle surface and interparticles lack a conductive network, preventing electrons from effectively transferring to the active material to participate in electrochemical reactions. Therefore, its theoretical capacity cannot be realized, resulting in extremely high internal resistance, and consequently, poor rate performance and cycle life. In-situ carbon coating is absolutely necessary to overcome the bottleneck of LiFePO4's inherently low electronic conductivity and to enable it to possess practical electrochemical performance. The ethylene cracking carbon coating process of this invention is crucial in this regard.

[0071] Eight comparative examples were carbon-coated without silicon modification. While exhibiting excellent material properties, they were slightly inferior to the optimal example. Their performance was very close to or even slightly better than some examples, and significantly better than most comparative examples. However, their 500-cycle retention was slightly lower than Example 1. Carbon coating successfully constructed a highly efficient electronic conductivity network, ensuring excellent capacity and rate performance. However, the lack of amorphous SiO2 thin-layer modification generated from the pyrolysis of tetraethoxysilane (TEOS) allowed the particle surface to directly contact the electrolyte, potentially leading to more pronounced interfacial side reactions during long-term cycling. This resulted in a slow increase in interfacial impedance and loss of active lithium, thus affecting the absolute stability during long-term cycling. Carbon coating is the main driver of performance improvement, while SiO2 modification is an enhancement. The composite gas surface modification strategy of ethylene and TEOS in this invention achieves high electronic conductivity while further stabilizing the electrode / electrolyte interface, resulting in optimal overall performance, particularly demonstrating advantages in long-term cycling stability.

[0072] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, characterized in that, Includes the following steps: (1) Purification treatment: After dissolving the titanium dioxide byproduct ferrous sulfate, add disodium ethylenediaminetetraacetate, citric acid, ascorbic acid and polyacrylamide as a composite purifying agent for purification treatment. After the reaction, filter to obtain a purified ferrous sulfate solution. (2) Electrochemical control: The purified ferrous sulfate solution is used as the electrolyte, and Fe is controlled by electrochemical methods. 2+ Partially oxidized to Fe 3+ Phosphoric acid was then added to the resulting mixed-valence iron solution, and the pH was adjusted using ammonia. After aging, an iron phosphate precursor slurry was formed. In step (2), the working electrode of the three-electrode system was a titanium-based IrO2-Ta2O5 coated anode, with a controlled potential of +0.7-0.8V vs. Ag / AgCl, and the Fe... 2+ The oxidation ratio is 20-40%; (3) Microwave synthesis: The iron phosphate precursor slurry and the lithium hydroxide ethanolamine-water mixed solution are mixed by a microfluidic mixer and then reacted in a microwave reactor to obtain a lithium iron phosphate precursor suspension; (4) Drying and crystallization: The lithium iron phosphate precursor is separated, dried by supercritical CO2, and then crystallized in a fluidized bed reactor. Simultaneously, carbon source and silicon source vapors are introduced for gas-phase surface modification to obtain lithium iron phosphate material.

2. The method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, is characterized in that... In step (1), the composite purifying agent comprises 10% by mass of disodium ethylenediaminetetraacetate solution, 15% by mass of citric acid solution, 5% by mass of ascorbic acid solution and 0.5% by mass of polyacrylamide solution.

3. The method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, is characterized in that... The purification reaction conditions are: pH 3.5-4.0, temperature 55-65℃, and time 1.5-2.5 hours.

4. The method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, is characterized in that... In step (2), the amount of phosphoric acid added is 1.01-1.05 times the total molar amount of iron in the solution. The pH of the mixed solution is adjusted to 2.5-3.0 using ammonia water, and aged at 50-70℃ for 0.5-1.5 hours.

5. The method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, characterized in that, In step (3), the lithium hydroxide ethanolamine-water mixed solution has a lithium hydroxide concentration of 1.8-2.2 mol / L and a volume ratio of ethanolamine to water of 1:3-1:5; the molar ratio of lithium source to iron (Li:Fe) is 1.01-1.05:

1.

6. The method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, is characterized in that... The conditions for the microwave reaction in step (3) are: power 500-1000W, reaction temperature 115-135℃, pressure 0.4-0.8MPa, and residence time 10-15 minutes.

7. The method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, characterized in that, The conditions for supercritical CO2 drying in step (4) are: pressure 8-12 MPa, temperature 35-45℃, and time 3-5 hours.

8. The method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide production, according to claim 1, characterized in that, The crystallization conditions in step (4) are: nitrogen atmosphere, temperature 430-470℃, time 1.5-2.5 hours; the gas phase surface modification includes simultaneously introducing ethylene as a carbon source and tetraethoxysilane vapor as a silicon source.

9. The application of the method for preparing lithium iron phosphate using ferrous sulfate, a byproduct of titanium dioxide, as described in any one of claims 1-8, in lithium-ion batteries.

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