A method for preparing sodium vanadium fluorophosphate, a cathode material for sodium-ion batteries, using electrolytic aluminum waste, and the cathode material itself.

Abstract

This invention relates to the field of electrochemical energy storage material preparation technology, specifically to a method and the cathode material, sodium vanadium fluorophosphate, for preparing sodium-ion batteries using electrolytic aluminum waste. The invention uses cryolite and vanadium alumina slag from electrolytic aluminum waste as raw materials. Through a process of "activation-synergistic leaching-deep selective impurity removal-precise fluoride-phosphorus control," sodium, vanadium, and fluorine resources are extracted and purified. High-performance products are synthesized through temperature-controlled seed crystal induction. Aluminum is thoroughly removed through solvent extraction / ion exchange, and the ratio is precisely controlled by an online fluoride ion monitoring system, solving the problems of complex impurities and large compositional fluctuations in the waste material. The Na3V2(PO4)2F3 prepared by this invention has a purity ≥99.2%, an iron content <30ppm, regular crystals, uniform particle size, a first discharge specific capacity ≥126mAh / g at 0.1C, and a capacity retention rate ≥92% after 500 cycles at 1C, demonstrating excellent performance.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage material preparation technology, specifically to a method for preparing sodium vanadium fluorophosphate, a cathode material for sodium-ion batteries, using electrolytic aluminum waste, and the cathode material itself. Background Technology

[0002] Sodium-ion batteries, due to their abundant resources and high cost potential, have shown great promise in the field of large-scale energy storage. Among the many cathode materials for sodium-ion batteries, sodium vanadium fluorophosphate (Na3V2(PO4)2F3, NVPF) has attracted much attention due to its high operating voltage, stable three-dimensional NASICON-type framework structure, and moderate theoretical specific capacity. However, its commercial application is limited by the cost of raw materials (especially vanadium and fluorine sources) and the complexity of the preparation process. Currently, the preparation of NVPF mainly relies on high-purity chemical raw materials. For example, patent CN103022490A discloses a method for preparing sodium vanadium fluorophosphate, a cathode material for sodium-ion batteries. This method uses a solid-state method, in which V2O5, HF, NH4H2PO4, and Na2CO3 are ball-milled and then calcined at high temperature to obtain the product. This method is simple, but the raw materials are expensive, and fluorine is easily lost at high temperatures, resulting in poor product uniformity. Patent CN108682855A discloses a method for controllable preparation of sodium vanadium fluorophosphate cathode material. This method uses a hydrothermal method with NH4VO3, H3PO4 and NaF as precursors. Although it can obtain nanoparticles, the process is lengthy, the preparation cost of sodium vanadate is high, and it still uses high-purity sodium fluoride. These methods have not solved the fundamental problem of high raw material costs and have failed to achieve resource recycling.

[0003] On the other hand, the electrolytic aluminum industry generates a large amount of hazardous solid waste. Among them, waste cryolite (mainly composed of Na3AlF6) is a waste generated during the overhaul of aluminum electrolytic cells. It is rich in sodium and fluorine. Currently, the main attempts are to return it to the electrolytic cell or use it to produce fluoride salts, but impurities limit its application, and a large amount of waste is forced to be stockpiled. In the alumina production process, vanadium-containing tailings or intermediate products are generated after vanadium slag is extracted from vanadium slag. It usually exists in the form of ammonium metavanadate, but it still contains a lot of impurities, and its high-value utilization is limited. Co-converting these two types of waste into high-value-added battery materials can not only reduce the cost of NVPF, but also solve the environmental problems of the aluminum industry, but it faces the following technical challenges: 1) The waste composition is complex and fluctuates greatly, with high impurity content (such as Fe, Si, Ca, Al). Due to the difficulty in completely separating aluminum impurities, aluminum ions are very easy to co-precipitate with vanadium or enter the crystal lattice, which seriously degrades the material performance; 2) The leaching rate and stoichiometry of fluorine in the waste are uncontrollable, which leads to deviations in the stoichiometry of the final product and affects the structural stability; 3) The fluctuation of waste composition poses a severe test to the process stability and product consistency.

[0004] Currently, there are no patents or literature reports on the direct preparation of high-performance sodium vanadium fluorophosphate cathode materials using electrolytic aluminum waste cryolite and vanadium alumina slag as the main raw materials. Summary of the Invention

[0005] To address the technical problems of high raw material costs in the preparation of existing sodium vanadium fluorophosphate cathode materials and the difficulty in utilizing waste cryolite and vanadium alumina slag from electrolytic aluminum in a high-value manner, this invention provides a method and cathode material for preparing sodium vanadium fluorophosphate cathode material for sodium-ion batteries using waste electrolytic aluminum. The aim is to achieve high-value utilization of waste cryolite and vanadium alumina slag from electrolytic aluminum while preparing sodium vanadium fluorophosphate cathode material with stable performance for sodium-ion batteries.

[0006] The technical solution of this invention is as follows: In a first aspect, the present invention provides a method for preparing sodium vanadium fluorophosphate, a cathode material for sodium-ion batteries, using electrolytic aluminum waste, comprising the following steps: (a) Raw material pretreatment and vanadium leaching: Vanadium-containing alumina waste slag is reduced and roasted, and the roasted product is acid-leached to obtain a vanadium-containing main solution; waste cryolite is wet-milled and / or activated by low-temperature roasting. The reduction roasting activates the vanadium leaching activity in the vanadium slag, and the low-temperature activation / wet milling breaks down the dense structure of the waste cryolite, achieving efficient release of sodium and fluorine resources and improving the leaching efficiency of effective components in the waste.

[0007] (b) Synergistic leaching and primary purification: The activated waste cryolite is mixed with the vanadium-containing main solution and reacted under heating and pH control conditions to achieve simultaneous leaching of sodium, fluorine and vanadium and preliminary precipitation of some impurities. After solid-liquid separation, a mixed solution containing Na, V, Al and F is obtained.

[0008] (c) Deep selective purification: The mixed solution obtained in step (b) is subjected to deep purification, which includes at least the following steps: (c1) Reduction-oxidation for iron removal: Sulfite is added to the mixed solution obtained in step (b) under stirring conditions, and the pH is controlled to carry out the reduction reaction; subsequently, the system is heated, and 3% H2O2 solution is slowly added dropwise or air is introduced, while the pH is adjusted with dilute NaOH, and the precipitate is aged by keeping it at the temperature. Finally, hot filtration is performed. Precise control of reaction conditions enables efficient reduction of Fe. 3+ Post-oxidation precipitation achieves deep removal of iron and some heavy metal impurities, with the iron content ultimately below 30 ppm, ensuring the electrochemical performance of the material. (c2) Selective aluminum removal: Selective aluminum separation is performed on the solution after step (c1) by solvent extraction or ion exchange to selectively remove aluminum ions from the solution and obtain a high-purity purified solution containing Na, V and F.

[0009] (d) Precursor synthesis and fluoride / phosphorus regulation: Phosphate was added to the purified liquid obtained in step (c). The required fluoride source was calculated and replenished via online fluoride ion concentration monitoring to precisely adjust the F / V molar ratio of the system to (1.45-1.55):1. Subsequently, the pH of the system was adjusted to 2.0-4.0 to initiate a co-precipitation reaction, yielding the precursor slurry. Real-time monitoring and precise fluoride replenishment ensured a stable stoichiometric ratio of the product, resolving the issue of uncontrollable fluoride leaching from the waste material and preventing structural stability from being affected by deviations in fluoride content.

[0010] (e) Temperature-controlled crystallization: The precursor slurry obtained in step (d) is subjected to crystallization treatment, and sodium vanadium fluorophosphate seed crystals are added during the crystallization process to induce the generation of precursors with complete crystal form and uniform particle size. Seed induction combined with slow temperature-controlled cooling promotes complete crystal growth, achieves uniform product particle size, and solves the problem of uneven crystal morphology caused by fluctuations in waste composition.

[0011] (f) Carbon coating and sintering: The precursor obtained in step (e) is mixed with a carbon source and sintered in an inert or reducing atmosphere to obtain a carbon-coated sodium fluorophosphate composite material.

[0012] Furthermore, in step (a), the conditions for the reduction roasting of the vanadium-containing alumina waste residue are: roasting at 500-650℃ in an inert or weakly reducing atmosphere for 1-3 hours. This temperature range and atmosphere conditions can efficiently activate the chemical activity of vanadium in the vanadium-containing alumina waste residue, converting the sparingly soluble vanadium into an easily acid-leached valence state, significantly improving the vanadium leaching rate, providing a sufficient and stable vanadium source for the subsequent preparation of high-purity sodium vanadium fluorophosphate, while avoiding energy waste or waste residue sintering and agglomeration caused by excessively high temperatures.

[0013] Furthermore, in step (a), the low-temperature roasting activation conditions for the waste cryolite are: roasting at 300-450℃ for 1-2 hours. Low-temperature roasting can destroy the dense crystal structure of the waste cryolite, reduce its lattice energy, and at the same time avoid the loss of fluorine due to high temperature. Combined with wet grinding, it can further improve the dissolution efficiency of sodium and fluorine in subsequent reactions, ensuring the full release of effective resources in the waste.

[0014] Furthermore, in step (c1), the sulfite is sodium sulfite or ammonium sulfite, and its addition amount is based on SO3. 2- The calculation is to reduce all Fe in the theoretically reduced solution. 3+ The amount of H2O2 added should be 1.5-3.0 times the molar amount required for the precipitation of heavy metal ions; the pH value for the reduction reaction should be 2.5-3.0; the reaction time should be 30 minutes; the amount of H2O2 added should be the theoretical amount for the oxidation of Fe. 2+The required molar amount is 1.0-1.2 times the normal amount. The final pH of the oxidation reaction is controlled at 4.5-5.2, the reaction temperature is 80-95℃, and the aging time is 1-2 hours. Preferably, 1-5 ppm of polyacrylamide flocculant can be added during the aging and precipitation to promote precipitate coagulation. Precise control of parameters such as reagent dosage, pH value, and temperature is crucial. Fe can be pre-treated... 3+ Efficient reduction to Fe 2+ Then, through an oxidation reaction, it is converted into a stable ferric hydroxide precipitate, while removing some heavy metal impurities; the addition of polyacrylamide flocculant can accelerate precipitation and coagulation, improve solid-liquid separation efficiency, and ultimately reduce the iron content in the product to less than 30 ppm, thus avoiding the impact of iron impurities on the electrochemical performance of the material.

[0015] Furthermore, in step (c2), the solvent extraction method uses a kerosene solution of phosphoric acid extractant P204 or P507, and performs multi-stage countercurrent extraction under pH conditions of 2.5-4.0 to achieve efficient separation of vanadium and aluminum; the ion exchange method uses Al... 3+ Highly selective chelating resins. P204 and P507 extractants and highly selective chelating resins are highly targeted to aluminum ions, and can achieve efficient separation of vanadium and aluminum under specific pH conditions, with an aluminum removal rate of over 99%, solving the problem of aluminum impurities easily co-precipitating with vanadium or intruding into the crystal lattice.

[0016] Further, in step (d), the phosphate is one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or ammonium phosphate; the molar ratio of the added phosphate to the vanadium in the purification solution is (1.0-1.1):1; the online fluoride ion concentration monitoring uses a fluoride ion selective electrode and is connected to an automatic control system. Based on the real-time monitored fluoride concentration and the target stoichiometric ratio, the system automatically calculates and adds a supplementary fluoride source, which is a NaF or NH4F solution. The selection of easily soluble and highly reactive phosphates, combined with precise control of the phosphorus-vanadium molar ratio, ensures sufficient and appropriate phosphorus content in the precursor. The online fluoride ion monitoring and automatic fluoride replenishment system can correct the uncertainty of fluoride leaching from the waste material in real time, precisely controlling the F / V molar ratio at (1.45-1.55):1, avoiding deviations in the product's stoichiometric ratio due to fluoride content variations, and ensuring the stability of the material structure.

[0017] Furthermore, in step (e), the cooling rate is 0.5-1.5℃ / min, and the cooling range is from 85℃ to -45℃; the amount of seed crystal added is 0.5%-2.0% of the theoretical yield of the precursor, and the addition is made when the system temperature drops to 50-55℃.

[0018] Furthermore, in step (f), the amount of organic carbon source added is 5-25 wt% of the precursor mass, and the sintering adopts a two-stage heating program: first, the temperature is raised to 350-450℃ at a rate of 3-5℃ / min and held for 1-2 hours to fully decompose the precursor and carbon source, and then raised to 650-750℃ at the same rate and held for 4-8 hours for crystallization; or microwave sintering is used, holding at 650-720℃ for 20-60 minutes. The two-stage sintering can first fully decompose the precursor and carbon source, and then ensure sufficient crystal growth.

[0019] Secondly, the present invention provides a positive electrode material prepared by the above method, which has the chemical formula Na3V2(PO4)2F3, a purity ≥99.2%, an iron content of less than 30ppm, aluminum in the form of dopant with a content of 0.5-1.5 at.%, and crystals that are regular polyhedral particles with a particle size distribution D50 of 200-600nm.

[0020] Furthermore, the initial discharge specific capacity at 0.1C rate is ≥126mAh / g, and the capacity retention rate after 500 cycles at 1C rate is ≥92%.

[0021] The beneficial effects of this invention are as follows: 1. This invention achieves efficient leaching of effective components from waste materials through a simultaneous reduction-dissolution vanadium-fluoride sodium release technology. This invention solves the problem of deep aluminum ion removal by combining sulfite reduction-oxidation with selective solvent extraction / ion exchange in a deep purification process. Furthermore, this invention overcomes the influence of waste material composition fluctuations by precisely controlling the stoichiometry and crystal morphology of the product through online fluoride ion monitoring and feedback and programmed temperature-controlled seeding technology.

[0022] 2. This invention utilizes cryolite from electrolytic aluminum waste and vanadium alumina slag as raw materials to achieve high-value utilization of solid waste, reduce the preparation cost of sodium vanadium fluorophosphate, and produce a product with a first discharge specific capacity of ≥126mAh / g at 0.1C and a capacity retention rate of ≥92% after 500 cycles at 1C, thus achieving both economic and environmental benefits. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a process flow diagram of the preparation process in Examples 1-2 of the present invention.

[0025] Figure 2 This is the XRD pattern of sodium vanadium fluorophosphate obtained in Example 1 of this invention.

[0026] Figure 3 This is a SEM image of sodium vanadium fluorophosphate prepared in Example 1 of this invention.

[0027] Figure 4 This is a charge-discharge cycle diagram of sodium vanadium fluorophosphate prepared in Example 1 of the present invention.

[0028] Figure 5 This is the XRD pattern of sodium vanadium fluorophosphate obtained in Example 2 of this invention.

[0029] Figure 6 This is a SEM image of sodium vanadium fluorophosphate prepared in Example 2 of this invention.

[0030] Figure 7 This is a charge-discharge cycle diagram of sodium vanadium fluorophosphate prepared in Example 2 of the present invention.

[0031] Figure 8 This is a charge-discharge cycle diagram of sodium vanadium fluorophosphate prepared in Comparative Example 1 of this invention.

[0032] Figure 9 This is a SEM image of sodium vanadium fluorophosphate prepared in Comparative Example 2 of this invention.

[0033] Figure 10 This is a charge-discharge cycle diagram of sodium vanadium fluorophosphate prepared in Comparative Example 2 of this invention. Detailed Implementation

[0034] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0035] Example 1 The main components of alumina vanadium slag are: V2O5 15.2%, Fe2O3 8.5%, and Al2O3 22.1%; the main components of electrolytic aluminum waste cryolite are: Na3AlF5 78.3%, and the main impurities are Al2O3 and CaF2.

[0036] (a) Vanadium slag pretreatment and leaching: Take 1200g of alumina vanadium slag, mix it evenly with 120g of activated carbon powder, place it in a tube furnace, and calcine it at 600℃ for 2 hours under N2 atmosphere. After cooling, leach it with 3mol / L sulfuric acid solution (liquid-solid ratio 5:1) at 90℃ for 2.5 hours, filter it, and obtain about 6L of vanadium-containing main solution.

[0037] (b) Activation and synergistic leaching of waste cryolite: Take 150g of waste cryolite, of which 75g is added to 400mL of hot water at 70℃ and wet ball milled for 1.5 hours to obtain ball milling slurry. The other 75g of waste cryolite is calcined at 350℃ for 1.5 hours and then mixed with the ball milling slurry. This slurry is then added to the vanadium-containing main solution in step (a), the pH is adjusted to 2.5 with dilute NaOH, and the reaction is stirred at 85℃ for 3 hours. The mixture is then filtered while hot to obtain about 6L of mixed filtrate.

[0038] (c) Deep selective purification: The mixed solution obtained in step (b) is subjected to deep purification, first by a reduction-oxidation step to remove iron, followed by a selective aluminum removal step: (c1) Reduction-oxidation to remove iron: Add 240g of solid Na2SO3 (approximately 1.9mol SO3) in batches to the mixture obtained in step (b) under stirring. 2- Adjust the pH to 2.5-3.0 and react for 30 minutes. Then, heat the system to 90℃ and slowly add a 3% H2O2 solution. The amount of H2O2 added is equal to the theoretical amount needed to oxidize Fe. 2+ Add 1.0-1.2 times the required molar amount, controlling the addition to stabilize the pH at the reaction endpoint at 5.0. After addition, incubate at 90℃ for 2 hours, then hot filter to obtain approximately 5.8L of clear and transparent purified liquid.

[0039] (c2) Selective aluminum removal: The pH of the purified solution from (c1) was adjusted to 3.0 with 10% sulfuric acid, and a three-stage countercurrent extraction was performed using a 15% P2O4-kerosene solution. The aqueous phase was the high-purity purified solution. ICP analysis showed that the Al content was <8 ppm, Fe <25 ppm, and V retention rate was >98%.

[0040] (d) Fluorine and phosphorus regulation and precursor synthesis: The fluorine concentration in the purified solution (5.8 L) was measured to be 0.47 mol / L using a fluoride ion electrode, indicating a total fluorine content of approximately 2.73 mol. Based on the stoichiometric ratio of the target product Na3V2(PO4)2F3 (F / V = 1.5) and the total vanadium content in the solution (approximately 1.97 mol), the required total fluorine content was calculated to be approximately 2.95 mol. Therefore, 0.22 mol of fluorine needed to be added. Accordingly, approximately 9.2 g (0.22 mol) of analytical grade sodium fluoride (NaF) was added to the system and stirred until completely dissolved, adjusting the fluorine concentration to approximately 0.51 mol / L. Then, 285 g (approximately 2.1 mol) of (NH4)2HPO4 was added and stirred until dissolved. The pH of the system was then slowly adjusted to 3.0 using 10% ammonia, resulting in a pale yellow precipitate.

[0041] (e) Temperature-controlled crystallization: The precipitated slurry was transferred to a crystallization vessel and cooled from 85°C to 52°C at a rate of 1.0°C / min. 4g of NVPF seed crystals were added, and the temperature was further reduced to 45°C for 3 hours of aging. After filtration and aging, the slurry was washed with deionized water and ethanol, and then dried at 100°C for 12 hours to obtain the precursor powder.

[0042] (f) Carbon coating and microwave sintering: The precursor was mixed with 15wt% glucose and placed in a microwave sintering furnace. Under N2 atmosphere, it was kept at 720℃ for 40 minutes and then naturally cooled to obtain carbon-coated sodium vanadium fluorophosphate black powder product, weighing 414g.

[0043] Performance testing: The sodium vanadium fluorophosphate product prepared in Example 1 of this invention was subjected to XRD analysis and SEM image analysis. The XRD pattern and SEM images are shown below. Figure 2 , Figure 3 As shown. By Figure 2 It can be seen that the material is a pure phase Na3V2(PO4)2F3, without impurity peaks, and has good crystallinity; Figure 3 It can be seen that the product has an irregular polyhedral morphology with a particle size of about 300-600 nm and a uniform distribution.

[0044] The initial charge and discharge capacities were 117.3 and 109.3 mAh / g, respectively, and after 100 cycles, the charge and discharge capacities were 103.2 and 101.7 mAh / g, respectively, demonstrating good electrochemical performance. Figure 4 As shown.

[0045] ICP test results show: purity 99.5%, Fe: 22ppm, Al: 0.9 at.%, Si: 60ppm, Ca: 98ppm.

[0046] Example 2 The alumina vanadium slag was taken from another production batch. The main components of the alumina vanadium slag are: V2O5 13.1%, Fe2O3 7.2%, and Al2O3 25.3%. The electrolytic aluminum waste cryolite came from different electrolytic cells. Its main components are: Na3AlF5 65.5%, and the content of Al2O3 and SiO2 among the impurities is relatively high.

[0047] step: (a) Vanadium slag pretreatment and leaching: Follow the steps in Example 1.

[0048] (b) Activation and synergistic leaching of waste cryolite: The procedure was carried out in accordance with the steps of Example 1. The content of aluminum and silicon impurities in the resulting mixed solution was significantly higher than that in Example 1.

[0049] (c) Depth-selective decontamination: The mixed solution obtained in step (b) is subjected to deep purification, first by a reduction-oxidation iron removal step, followed by a selective aluminum removal step: (c1) Reduction precipitation to remove iron: Add 205g of solid Na2SO3 (approximately 1.6mol SO3) in batches to the mixture obtained in step (b) under stirring. 2- Adjust the pH to 2.5-3.0 and react for 30 minutes. Then, heat the system to 90℃ and slowly add a 3% H2O2 solution. The amount of H2O2 added is equal to the theoretical amount needed to oxidize Fe. 2+ Add 1.0-1.2 times the required molar amount, controlling the addition to stabilize the pH at the reaction endpoint at 5.0. After addition, incubate at 90℃ for 2 hours, then hot filter to obtain approximately 5.8L of clear and transparent purified liquid.

[0050] (c2) Selective extraction for aluminum removal: Given the high aluminum content, the pH of the filtrate was adjusted to 3.2, and a four-stage countercurrent extraction was performed using an 18% P507-sulfonated kerosene solution to enhance separation. After extraction, the aluminum content in the aqueous phase decreased from >1200ppm to <5ppm, and the vanadium loss rate was <2%.

[0051] (d) Fluorine and phosphorus regulation and precursor precipitation: The fluorine concentration in the purified solution (5.8 L) was measured to be 0.40 mol / L using a fluoride ion electrode, indicating a total fluorine content of approximately 2.3 mol. Based on the stoichiometric ratio of the target product Na3V2(PO4)2F3 (F / V = 1.5) and the total vanadium content in the solution (approximately 1.69 mol), the required total fluorine content was calculated to be approximately 2.59 mol. Therefore, 0.29 mol of fluorine needed to be added. Accordingly, approximately 10.7 g (0.29 mol) of analytical grade ammonium fluoride (NH4F) was added to the system, stirred until completely dissolved, and the fluorine concentration was adjusted to approximately 0.45 mol / L. Then, 246 g of (NH4)2HPO4 was added, and the pH was adjusted to 3.0 with ammonia water, resulting in precipitation.

[0052] (e) Temperature-controlled crystallization: Performed according to the steps in Example 1.

[0053] (f) Carbon coating and sintering: Following the steps of Example 1, a two-stage sintering process was adopted, with a holding temperature of 400°C for 1.5 hours and 700°C for 6 hours. After natural cooling, a carbon-coated sodium vanadium fluorophosphate black powder product was obtained, weighing 358g.

[0054] Performance testing: The sodium vanadium fluorophosphate product prepared in Example 2 of this invention was subjected to XRD analysis and SEM morphology analysis. The XRD pattern and SEM image are shown below. Figure 5 , Figure 6 As shown. By Figure 5It can be seen that the material is a pure phase Na3V2(PO4)2F3, without impurity peaks, and has good crystallinity; Figure 6 It can be seen that the product has a granular morphology with regular and consistent appearance, and an average particle size D50 of about 580 nm.

[0055] The initial charge and discharge capacities were 116 and 108.6 mAh / g, respectively, and after 100 cycles, the charge and discharge capacities were 104 and 102.3 mAh / g, respectively, demonstrating good electrochemical performance. Figure 7 As shown.

[0056] ICP test results show: purity 99.3%, Fe: 27ppm, Al: 1.1at.% (controlled doping).

[0057] Comparative Example 1 Comparative Example 1 uses the traditional solid-state method, with high-purity V2O5, NaF, NH4H2PO4, and Na2CO3 as raw materials. They are ball-milled and mixed in stoichiometric ratio, and sintered at 750°C for 10 hours in a tube furnace under N2 atmosphere.

[0058] Performance testing: Test results: The obtained sample had a purity of 99.6%, a capacity of 126 mAh / g at 0.1C, and a retention rate of 88.5% after 500 cycles at 1C. Furthermore, the raw material cost was high. (The charge-discharge cycle spectrum of Comparative Example 1 is shown below.) Figure 8 (As shown).

[0059] Comparative Example 2 Performance testing: Test results: The obtained sample had a purity of 96.8%, Fe: 305 ppm, and Al: 2.8 at.%. The particle morphology was uneven, with some agglomeration. The 1C capacity was 91 mAh / g, and the retention rate after 160 cycles was only 53.2%. Performance was severely compromised. (SEM images and charge-discharge cycle spectra of Comparative Example 2 are shown below.) Figure 9 , Figure 10 (As shown).

[0060] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for preparing sodium vanadium fluorophosphate, a cathode material for sodium-ion batteries, using electrolytic aluminum waste, characterized in that, Includes the following steps: (a) Raw material pretreatment and vanadium leaching: Vanadium-containing alumina waste residue is reduced and roasted, and the roasted product is acid-leached to obtain vanadium-containing main solution; waste cryolite is wet-milled and / or activated by low-temperature roasting; (b) Synergistic leaching and primary purification: The activated waste cryolite is mixed with the vanadium-containing main solution and reacted under heating and pH control conditions to achieve simultaneous leaching of sodium, fluorine and vanadium and preliminary precipitation of some impurities. After solid-liquid separation, a mixed solution containing Na, V, Al and F is obtained. (c) Deep selective purification: The mixed solution obtained in step (b) is subjected to deep purification, which includes at least the following steps: (c1) Reduction-oxidation to remove iron: Sulfite is added to the mixed solution obtained in step (b) under stirring conditions, and the pH is controlled to carry out the reduction reaction; then, the system is heated, 3% H2O2 solution is slowly added dropwise or air is introduced, while the pH is adjusted with dilute NaOH, the precipitate is kept warm and matured, and finally hot filtration is performed. (c2) Selective aluminum removal: Selective aluminum separation is performed on the solution after step (c1) by solvent extraction or ion exchange to selectively remove aluminum ions from the solution and obtain a high-purity purified solution containing Na, V and F. (d) Precursor synthesis and fluoride-phosphorus regulation: Add phosphate to the purified liquid obtained in step (c), calculate and replenish the required fluoride source by online monitoring of fluoride ion concentration to accurately adjust the F / V molar ratio of the system to (1.45-1.55):1, and then adjust the pH of the system to 2.0-4.0 to initiate a co-precipitation reaction to obtain the precursor slurry; (e) Temperature-controlled crystallization: The precursor slurry obtained in step (d) is subjected to crystallization treatment, and sodium vanadium fluorophosphate seed crystals are added during the crystallization process to induce the generation of precursor; (f) Carbon coating and sintering: The precursor obtained in step (e) is mixed with a carbon source and sintered in an inert or reducing atmosphere to obtain a carbon-coated sodium fluorophosphate composite material.

2. The method as described in claim 1, characterized in that, In step (a), the conditions for reducing and roasting the vanadium-containing alumina waste residue are: roasting at 500-650℃ in an inert or weakly reducing atmosphere for 1-3 hours.

3. The method as described in claim 1, characterized in that, In step S1, the low-temperature roasting activation conditions for the waste cryolite are: roasting at 300-450℃ for 1-2 hours.

4. The method as described in claim 1, characterized in that, In step (c1), the sulfite is sodium sulfite or ammonium sulfite, and the amount added is based on SO3. 2- The calculation is to reduce all Fe in the theoretically reduced solution. 3+ The amount of H2O2 added should be 1.5-3.0 times the molar amount required for the precipitation of heavy metal ions; the pH value for the reduction reaction should be 2.5-3.0; the reaction time should be 30 minutes; the amount of H2O2 added should be the theoretical amount for the oxidation of Fe. 2+ The required molar amount is 1.0-1.2 times, the pH at the end of the oxidation reaction is controlled at 4.5-5.2, the reaction temperature is 80-95℃, and the heat preservation and ripening time is 1-2h.

5. The method as described in claim 1, characterized in that, In step (c2), the solvent extraction method uses a kerosene solution of phosphoric acid extractant P204 or P507, and performs multi-stage countercurrent extraction under pH conditions of 2.5-4.0 to achieve efficient separation of vanadium and aluminum; the ion exchange method uses Al... 3+ A highly selective chelating resin.

6. The method as described in claim 1, characterized in that, In step (d), the phosphate is one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, or ammonium phosphate; the molar ratio of the added phosphate to the vanadium in the purification solution is (1.0-1.1):1; the online fluoride ion concentration monitoring uses a fluoride ion selective electrode and is connected to an automatic control system, which automatically calculates and adds a supplementary fluoride source based on the real-time monitored fluoride concentration and the target stoichiometric ratio, and the supplementary fluoride source is a NaF or NH4F solution.

7. The method as described in claim 1, characterized in that, In step (e), the cooling rate is 0.5-1.5℃ / min, and the cooling range is 85℃-45℃; the amount of seed crystals added is 0.5%-2.0% of the theoretical yield of the precursor, and the addition is made when the system temperature drops to 50-55℃.

8. The method as described in claim 1, characterized in that, In step (f), the amount of carbon source added is 5-25 wt% of the precursor mass; the sintering adopts a two-stage heating program: first, the temperature is raised to 350-450℃ at 3-5℃ / min and held for 1-2 hours to fully decompose the precursor and carbon source, and then the temperature is raised to 650-750℃ at the same rate and held for 4-8 hours for crystallization; or microwave sintering is adopted, and the temperature is held at 650-720℃ for 20-60 minutes.

9. A cathode material prepared by the method according to any one of claims 1-8, characterized in that, Its chemical formula is Na3V2(PO4)2F3, purity ≥99.2%, iron content less than 30ppm, aluminum exists in the form of dopant with an aluminum content of 0.5-1.5 at.%, the crystals are regular polyhedral particles with a particle size distribution D50 of 200-600nm.

10. The cathode material as described in claim 9, characterized in that, The initial discharge specific capacity at 0.1C rate is ≥126mAh / g, and the capacity retention after 500 cycles at 1C rate is ≥92%.

Images