Method for preparing high-performance sodium-ion battery positive electrode material by using waste lithium iron phosphate and application

Abstract

This invention provides a method and application for preparing high-performance sodium-ion battery cathode materials using waste lithium iron phosphate, comprising the following steps: mixing waste lithium iron phosphate with sulfuric acid at a molar ratio of 1:(0.5~1) for acid leaching to obtain a leachate; evaporating, concentrating, and crystallizing the leachate to obtain ferrous sulfate crystals; adding an oxidant to the filtrate and adjusting the pH to precipitate iron phosphate; further adjusting the pH to separate lithium phosphate; further adding sodium carbonate to obtain lithium carbonate; adding sulfuric acid to the remaining liquid to adjust the pH and then evaporating and crystallizing to obtain anhydrous sodium sulfate. Iron phosphate is pre-calcined and then mixed with a sodium source, phosphorus source, and carbon source for sintering to obtain high-performance sodium-ion battery cathode material phosphate; ferrous sulfate is pre-calcined and then mixed with anhydrous sodium sulfate and a carbon source for sintering to obtain high-performance sodium-ion battery cathode material sulfate. This invention achieves the recovery of lithium from waste lithium iron phosphate and simultaneously provides a technical path for synthesizing high-performance sodium-ion battery cathode material phosphate and high-performance sodium-ion battery cathode material sulfate.

Description

Technical Field

[0001] This invention relates to the field of battery waste material recycling technology, and in particular to a method and application for preparing high-performance sodium-ion battery cathode materials using waste lithium iron phosphate. Background Technology

[0002] With the widespread adoption of electric vehicles, a large number of lithium-ion batteries are entering their retirement period. Lithium iron phosphate (LFP) batteries occupy a significant market share due to their high safety and low cost, and their efficient recycling is crucial for resource sustainability and environmental protection. Existing recycling methods include pyrometallurgy, hydrometallurgy, and direct remanufacturing. Hydrometallurgy and pyrometallurgy typically aim to recover lithium and transition metals, but they suffer from problems such as complex processes, high acid and alkali consumption, heavy secondary pollution, or low product added value. Hydrometallurgy often uses acid and alkali solutions to leach lithium, and then separates and recovers lithium elements through precipitation, solvent extraction, and ion exchange. Low-value iron phosphate slag is often disposed of by landfill, resulting in significant resource waste. Current research indicates that the degradation of LFP cathodes is mainly attributed to the irreversible formation of Li vacancies, Fe vacancies, and Li-Fe antisite defects. These crystallographic defects gradually disrupt electron conduction pathways and hinder Li... + Diffusion kinetics ultimately lead to capacity decay. However, in sodium-ion battery Na4Fe3(PO4)2(P2O7) materials, defect design is considered an effective strategy to improve the electrochemical performance of the material, suppress the formation of impurity phases such as NaFePO4 and Na2FeP2O7, and mitigate structural collapse during fast charging and discharging. Furthermore, sodium-ion batteries, as a complement to lithium-ion batteries, have attracted widespread attention due to the abundance and low cost of sodium resources. In existing technologies, high-performance sodium-ion battery cathode materials such as phosphates (NFPP, e.g., sodium iron pyrophosphate: Na4Fe3(PO4)2(P2O7)) are commonly used as sodium-ion electronic cathode materials. However, using sodium iron pyrophosphate as a cathode material for sodium-ion batteries has problems such as low electronic conductivity, poor rate performance, and poor cycle performance at high rates. High-performance sodium-ion battery cathode materials such as sulfates (NFS, e.g., Na2Fe2(SO4)3) have become a research hotspot due to their excellent electrochemical performance and structural stability. They can significantly improve the rate performance of sodium-ion batteries. However, to achieve efficient, economical recovery of lithium from lithium iron phosphate that can be directly used as raw material for sodium-ion battery production, and to simultaneously synthesize high-performance sodium-ion battery cathode materials such as phosphates and sulfates, a technical path still needs to be established.

[0003] Therefore, developing an integrated process for preparing high-performance sodium-ion battery cathode materials using waste lithium iron phosphate, while simultaneously possessing a short process flow, high recycling rate, and high product value, has significant economic and environmental value. Summary of the Invention

[0004] This invention addresses the technical problem of low recycling rates of waste lithium iron phosphate (LFP) batteries by proposing a method that utilizes a defect inheritance strategy to transform the degradation pathway of waste LFP into the production of high-performance sodium-ion battery cathode materials, specifically phosphate and sulfate. On one hand, it achieves high-value recycling of waste LFP, recovering valuable lithium elements as Li3PO4 and a small portion as Li2CO3. The remaining recovered materials, FeSO4, FePO4, and Na2SO4, can be fully utilized in the preparation of high-performance sodium-ion battery cathode materials, forming an economically viable closed-loop technology from waste to raw materials and then to new products. On the other hand, the entire process generates no secondary pollution, has low energy consumption, and is green and efficient, achieving high-value recycling of waste LFP batteries and solving the problem of low recycling rates.

[0005] This invention is achieved through the following technical solution: In a first aspect, the present invention provides a method for preparing high-performance sodium-ion battery cathode materials using waste lithium iron phosphate, the preparation method comprising the following steps: S1: Waste lithium iron phosphate is mixed with sulfuric acid solution and subjected to acid leaching reaction to obtain leachate and insoluble matter; wherein the molar ratio of waste lithium iron phosphate to sulfuric acid is 1:(0.5~1). S2: The leachate obtained from S1 is evaporated, concentrated, cooled, and recrystallized to obtain ferrous sulfate crystals (FeSO4·xH2O), where 1≤x≤10; S3: Add an oxidant to the remaining filtrate after separating ferrous sulfate crystals in S2 to carry out an oxidation reaction, adjust the pH value of the solution, and separate to obtain ferric phosphate precipitate (FePO4·yH2O), where 1≤y≤10; S4: Add alkali to the remaining filtrate after separating the iron phosphate precipitate in S3 to adjust the pH value, and then separate lithium phosphate (Li3PO4). S5: Add sodium carbonate (Na2CO3) to the remaining filtrate after separating lithium phosphate in S4 to obtain lithium carbonate precipitate (Li2CO3); add sulfuric acid to the remaining filtrate after separating the lithium carbonate precipitate, adjust the pH value of the solution, and evaporate and crystallize to obtain anhydrous sodium sulfate (Na2SO4). S6: The iron phosphate precipitate (FePO4·yH2O) obtained in S3 is pre-sintered, mixed with sodium source, phosphorus source and carbon source, and sintered to prepare high-performance sodium-ion battery cathode material phosphate (NFPP); the FeSO4·xH2O obtained in S2 is pre-sintered, mixed with anhydrous sodium sulfate obtained in S5, and a carbon source is added. After sintering, high-performance sodium-ion battery cathode material sulfate (NFS) is obtained.

[0006] This invention provides a method for preparing high-performance sodium-ion battery cathode materials using waste lithium iron phosphate. The invention innovatively employs sulfuric acid as a strong acid to efficiently decompose the crystal structure of waste lithium iron phosphate, and specifies the molar ratio of waste lithium iron phosphate to sulfuric acid as 1:(0.5~1). This acid leaching reaction allows Li, Fe, and P elements in the lithium iron phosphate cathode material to enter the solution in ionic form. The insoluble matter is mostly conductive carbon, which can be filtered and recovered, and added as a partial carbon source for preparing high-performance sodium-ion battery cathode material sulfate and phosphate. At this acid dosage, the Fe, P, and Li elements in the waste lithium iron phosphate are fully released into the solution in ionic form, avoiding element recovery losses due to insufficient acid dosage or the introduction of impurity elements due to different acid types. More importantly, the specific molar amount of sulfuric acid used in this invention introduces sulfate ions (SO42-). 2- This directly provides the anionic basis for the subsequent stepwise crystallization and recovery of iron, and also provides the necessary sulfur source for the final synthesis of sulfate, a high-performance sodium-ion battery cathode material. This step realizes the transformation from solid waste to a controllable ionic solution, while simultaneously introducing key components, laying a precise chemical foundation for the subsequent integrated closed-loop process of "separation-synthesis". On the one hand, the acid leaching reaction defined in this invention, with a specific amount of sulfuric acid, provides a suitable crystallization environment for subsequent recrystallization, allowing it to preferentially crystallize Fe during the subsequent concentration process. 2+ The solution crystallizes in the form of ferrous sulfate crystals. Under the aforementioned acid leaching reaction, some iron is retained and, under suitable oxidation conditions, used to generate phosphate, a high-performance sodium-ion battery cathode material. Furthermore, the resulting suitable pH environment provides an initial pH environment and a pH-controlled pathway for subsequent pH adjustment to generate lithium phosphate (Li3PO4), achieving lithium recovery and enabling the synthesis of anhydrous sodium sulfate (Na2SO4), a sodium and sulfur source in the high-performance sodium-ion battery cathode material. This invention also employs a low-temperature evaporation crystallization strategy, concentrating the solution at low temperatures. The main purpose of this strategy is to increase the concentration of all solutes in the solution, especially Fe. 2+ and SO4 2- The product of the ion concentrations reaches or exceeds that of SO4. 2- The solubility product at the current temperature creates the driving force for crystallization, i.e., establishing a supersaturated state. Low-temperature evaporation also prevents the oxidation of iron ions. The solubility curve of FeSO4·xH2O exhibits a typical characteristic: its solubility decreases significantly with decreasing temperature. At higher temperatures, it remains soluble; when the solution is cooled, its solubility decreases sharply. By controlling the temperature of cooling recrystallization, the Fe in the solution can be guided... 2+ and SO4 2- Preferentially combining and forming crystals precipitate. Due to Li + phosphates and Li+ The solubility characteristics of sulfates under the same pH and temperature conditions (Li3PO4 is soluble under acidic conditions, and Li2SO4 has very high solubility) mean that they will remain in the solution, thus achieving the initial separation of Fe from Li and P. After some iron ions precipitate, the total iron content of the solution in subsequent steps is significantly reduced, while a certain amount of Fe is retained for the synthesis of phosphates for high-performance sodium-ion battery cathode materials. This reduces the processing load of subsequent oxidation and precipitation processes and makes the solution composition easier to control precisely. Therefore, the acid leaching technology combined with low-temperature recrystallization technology under the above acid leaching reaction conditions achieves the full recovery and combined utilization of waste lithium iron phosphate cathode materials, ensuring sufficient iron for Fe production. 2+ Ferrous sulfate is generated in the form of Fe to prepare sulfates for high-performance sodium-ion battery cathode materials, which also ensures that some iron can be generated as Fe. 3+ The form is used as a raw material for synthesizing phosphate, a positive electrode material for high-performance sodium-ion batteries.

[0007] Under the above-described method steps, this invention significantly solves the key problems existing in the recycling technology of waste lithium iron phosphate cathode materials, such as lengthy processes, large amounts of secondary waste, and low added value of recycled products. It provides a new method for short-process, high-value-added collaborative recycling and material regeneration. By designing a separation path for stepwise crystallization and controlled precipitation under specific acid leaching conditions, it overcomes the drawbacks of elemental interference and separation difficulties in traditional wet recycling. Abandoning the traditional approach of simply treating the recycled products as primary chemical products, the recycled products are positioned as high-quality precursors for high-performance sodium-ion battery cathode materials (phosphates), and the synthesis of two functionally complementary high-performance sodium-ion battery cathode materials is achieved. Through the targeted design and utilization of all output products in the process, the mother liquor containing Na2SO4 generated after the main recycling step is coupled with the FeSO4 separated in the previous step to synthesize another high-performance sodium-ion battery cathode material (sulfate), ultimately achieving near-complete resource utilization of the liquid phase components and reducing or even eliminating secondary wastewater discharge from the source. Transforming the vast resources of spent lithium iron phosphate batteries into key cathode materials for emerging sodium-ion batteries not only solves the problem of lithium battery recycling, but also provides an innovative raw material supply model for the low-cost and sustainable development of the sodium battery industry, promoting the synergy and circulation of the lithium battery and sodium battery industry chains.

[0008] As a further embodiment, S1 involves mixing waste lithium iron phosphate with a sulfuric acid solution of 0.5~5.0 mol / L at a stirring temperature of 25~80℃ and a stirring time of 1~5h to carry out an acid leaching reaction, thereby obtaining a leachate and insoluble matter, wherein the molar ratio of waste lithium iron phosphate to sulfuric acid is 1:(0.5~1). S2 involves evaporating and concentrating the leachate obtained in S1, then cooling and recrystallizing it to obtain ferrous sulfate crystals (FeSO4·xH2O). The heating temperature for evaporation and concentration is 60-80°C, the temperature for cooling and recrystallization is 5-15°C, and the settling time for cooling and recrystallization is 4-12 hours.

[0009] This invention further limits the concentration of the sulfuric acid solution used in the acid leaching reaction and conducts stirring at a higher temperature. Simultaneously, this invention further limits the heating temperature for evaporation and concentration, as well as the temperature and time for cooling and recrystallization. This further ensures the complete release of Fe, P, and Li elements into the solution in ionic form, providing a suitable ionic environment for the subsequent complete conversion and synthesis of the product. Furthermore, the further limited cooling and recrystallization time further allows Fe... 2+ and SO4 2- Preferential bonding and crystal precipitation are achieved. Under the above conditions, a more rational allocation and full utilization of elements are realized.

[0010] As a further option, the reaction equation for S1 can be expressed as: 2LiFePO4 + H2SO4 → 2Li + +2Fe 2+ + 2PO4 3- + SO4 2- + H2 .

[0011] As a further embodiment, the leachate in S1 is evaporated to 25% to 75% of the original solution volume.

[0012] As a further embodiment, the ferrous sulfate crystals (FeSO4·xH2O) are FeSO4·4H2O and / or FeSO4·7H2O.

[0013] As a further embodiment, S3 is to adjust the pH of the solution to 2-5, S4 is to add alkali to adjust the pH to 9-13, and S5 is to adjust the pH of the solution to 6.5-7.5.

[0014] This invention further defines the pH adjustment range for steps S3 to S5, thus ensuring the pH of PO4 in step S3 is controlled. 3- The concentration of Fe 3+ Achieving the solubility product of FePO4 ensures high selectivity and completeness of the precipitation reaction, while effectively inhibiting Fe... 3+ This forms colloidal hydroxides or other impurity phases. Simultaneously, it ensures that a large number of protons from the phosphate group in S4 are removed, reducing the PO4 content in the solution. 3-The concentration of sodium sulfate increases sharply, driving the precipitation reaction to form Li3PO4. Adjusting the pH to 6.5-7.5 in S5 ensures the full formation of sodium sulfate and avoids the residue of impurities such as sodium carbonate.

[0015] As a further option, the oxidant in S3 is sodium persulfate (Na2S2O8) and / or hydrogen peroxide (H2O2).

[0016] As a further preferred embodiment, the oxidant in S3 is sodium persulfate (Na2S2O8) and hydrogen peroxide (H2O2).

[0017] As a further preferred embodiment, the molar ratio of sodium persulfate to lithium iron phosphate is (0.5~1):2; the molar ratio of hydrogen peroxide to lithium iron phosphate is (0.1~0.4):1.

[0018] As a further preferred embodiment, step S3 involves adding sodium persulfate (Na2S2O8) and hydrogen peroxide (H2O2) to the remaining filtrate after separating ferrous sulfate crystals in step S2 for an oxidation reaction, wherein the molar ratio of sodium persulfate to lithium iron phosphate is (0.5~1):2; the molar ratio of hydrogen peroxide to lithium iron phosphate is (0.1~0.4):1; the pH of the solution is adjusted to 2.0~5.0, and ferric phosphate precipitate (FePO4·yH2O) is obtained by separation and washing.

[0019] The present invention further preferably uses sodium persulfate and hydrogen peroxide as oxidants for the oxidation reaction, and limits the amount of oxidants used. Sodium persulfate is a strong oxidant that can effectively oxidize Fe. 2+ Oxidized to Fe 3+ The key advantage lies in its relatively mild and stable oxidation reaction, and the fact that the byproduct produced by decomposition in aqueous solution is sodium sulfate, which is itself a component of the system and does not introduce new impurity ions, thus ensuring the purity of the subsequent products. H₂O₂ can react with Fe... 2+ The reaction generates hydroxyl radicals, which can rapidly initiate and significantly accelerate the entire oxidation process, especially in the early stages. The generated radicals can activate persulfate ions, promoting their decomposition to produce sulfate radicals, thus forming a dual radical oxidation system that greatly enhances oxidation efficiency and rate. Trace amounts of H₂O₂ and the resulting radicals can also oxidize and decompose any trace organic impurities in the solution (such as residues from battery binders), purifying the solution and facilitating the production of higher purity FePO₄.

[0020] As a further option, the concentration of the hydrogen peroxide solution is selected from 0.5% to 30%.

[0021] As a further embodiment, in step S3, an oxidant is added and the mixture is stirred at a speed of 100-1000 rpm to carry out an oxidation reaction. The oxidation reaction time is 0.5-2 hours, and the oxidation reaction temperature is 20-60°C.

[0022] As a further embodiment, the pH of the solution in S3 is adjusted to 2.0~3.0.

[0023] As a further example, after adjusting the pH in step S3, the precipitate is aged for 2 to 8 hours, and the precipitate includes FePO4·2H2O.

[0024] This invention can further limit the pH adjustment of the solution in S3, due to Fe 3+ With PO4 3- It can form a FePO4 precipitate with very low solubility. However, in acidic environments, PO4... 3- It will protonate to form H2PO4 - Or H3PO4, at very low concentrations. This invention further precisely controls the pH to 2.0–3.0 to balance the two: within this pH range, Fe… 3+ It has become stable and has begun to show signs of hydrolysis, while PO4 3- The concentration is also sufficient to match Fe 3+ This achieves the solubility product of FePO4. This further ensures high selectivity and completeness of the precipitation reaction, and effectively inhibits the precipitation of Fe. 3+ It forms colloidal hydroxides or other impurity phases.

[0025] As a further preferred embodiment, step S3 involves adding sodium persulfate (Na2S2O8) and hydrogen peroxide (H2O2) to the remaining filtrate after separating ferrous sulfate crystals in step S2, and carrying out an oxidation reaction at a stirring speed of 100-1000 rpm for 0.5-2 hours at a temperature of 20-60°C. The molar ratio of sodium persulfate to lithium iron phosphate is (0.5-1):2, and the molar ratio of hydrogen peroxide to lithium iron phosphate is (0.1-0.4):1. After adjusting the pH of the solution to 2.0-3.0, the solution is aged for 2-8 hours, and then ferric phosphate precipitate (FePO4·yH2O) is obtained by separation and washing.

[0026] This invention further optimizes the oxidant and its dosage used in the oxidation reaction in step S2, while also further limiting the pH adjustment. This further improves the efficiency of the oxidation reaction, thereby increasing the purity of the iron phosphate precipitate obtained in step S2 and further reducing the formation of other impurities.

[0027] As a further embodiment, the pH in S4 is adjusted to 10.5~11.5.

[0028] This invention further optimizes the pH adjustment range in step S4. When the pH is significantly raised to a strongly alkaline range of 10.5-11.5 by adding alkali, a large number of protons of phosphate are removed, and the PO4 in the solution... 3- The concentration of PO4 increases sharply, driving a precipitation reaction to form Li3PO4. If the pH is too low, PO4 will... 3- Insufficient concentration leads to incomplete precipitation; excessively high pH may introduce other impurities (such as forming trace amounts of LiOH or causing poor crystallinity of the precipitate) and increase alkali consumption. Under these strongly alkaline conditions, the main cation in the solution is Na. + and Li + Because Na3PO4 has extremely high solubility while Li3PO4 has very low solubility, this precipitation reaction is not effective for Li. + It exhibits high selectivity, with almost all sodium ions remaining in the solution, thus achieving effective separation of lithium from sodium and other potentially residual impurities.

[0029] As a further embodiment, the alkali in S4 is a NaOH solution with a concentration of 0.5~2.0 mol / L.

[0030] As a further step, after adjusting the pH in step S4, the cells are aged for 2 to 8 hours.

[0031] As a further step, step S4 involves adding 0.5-2.0 mol / L sodium hydroxide solution to the remaining filtrate after separating the iron phosphate precipitate in step S3 to adjust the pH to 10.5-11.5, followed by aging for 2-8 hours, separation, washing, and drying to obtain lithium phosphate (Li3PO4).

[0032] This invention further limits the pH adjustment value in step S4 and further limits the alkaline solution used to sodium hydroxide. This allows for better separation of lithium and sodium while reducing the introduction of other impurity elements, thus improving the purity of the obtained lithium phosphate.

[0033] As a further step, step S5 involves heating the remaining filtrate after separating lithium phosphate in step S4 to 60-80°C; and slowly adding Na2CO3 solution dropwise while continuously stirring until the pH value rises to 7.5-8.5.

[0034] This invention further specifies step S5 as adding a sodium carbonate solution under heating conditions. Heating to 60-80°C significantly reduces the solubility of Li₂CO₃, thereby greatly improving the lithium precipitation recovery rate and ensuring efficient extraction even of trace amounts of lithium. Simultaneously, increasing the temperature accelerates the reaction rate and improves the crystallinity of the precipitate, making it easier to filter. Precise pH control to 7.5-8.5 is necessary because of CO₃²⁻. 2- Hydrolysis occurs in aqueous solution. Maintaining a pH of 7.5–8.5 can help maintain CO3 levels in the solution.2- The effective concentration ensures the driving force for precipitation.

[0035] As a further embodiment, the concentration of the Na2CO3 solution in S5 is 0.02~1.0 mol / L.

[0036] As a further step, the pH value in S5 is raised to 7.5-8.5 and then aged for 1-4 hours.

[0037] As a further step, in step S5, sulfuric acid is added to the remaining filtrate after separating the lithium carbonate precipitate, and the pH is adjusted to 7. The mixture is then stirred at a rate of 200-250 rpm at a temperature of 50-60°C to slowly evaporate and crystallize anhydrous sodium sulfate (Na2SO4).

[0038] The present invention further specifies the addition of sulfuric acid to adjust the pH to 7 because after Li₂CO₃ precipitation, the mother liquor becomes alkaline due to the presence of excess Na₂CO₃. The addition of sulfuric acid initiates a neutralization reaction (Na₂CO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂↑), neutralizing the excess CO₃²⁻. 2- The process involves converting the sodium sulfate into CO2 gas, which escapes and generates the target product Na2SO4. This prevents the sodium sulfate from co-crystallizing with Na2SO4 during subsequent evaporation and crystallization, thus maintaining purity. Water is removed through evaporation, continuously increasing the concentration of Na2SO4 in the solution until it exceeds its solubility and crystallizes out. By controlling the evaporation rate and temperature, anhydrous sodium sulfate products with specific crystal forms and particle sizes can be obtained.

[0039] As a further preferred embodiment, in step S5, sulfuric acid with a concentration of 0.02~1.0 mol / L is added to the remaining filtrate after separating the lithium carbonate precipitate, and the pH is adjusted to 7. The mixture is then slowly evaporated and crystallized at a temperature of 50~60℃ and a stirring rate of 200~250 rpm to obtain anhydrous sodium sulfate (Na2SO4).

[0040] As a further preferred embodiment, the preparation method includes the following steps performed sequentially: S1: Waste lithium iron phosphate is mixed with a sulfuric acid solution with a concentration of 0.5~5.0 mol / L at a stirring temperature of 40~60℃ and a stirring time of 1~5h to carry out an acid leaching reaction, and a leachate and insoluble matter are obtained; wherein the molar ratio of waste lithium iron phosphate to sulfuric acid is 1:(0.5~1). S2: The leachate obtained from S1 is evaporated, concentrated, cooled, recrystallized, washed, and dried to obtain ferrous sulfate crystals (FeSO4·xH2O). The heating temperature for evaporation and concentration is 60-80°C, the temperature for cooling and recrystallization is 7-12°C, and the settling time for cooling and recrystallization is 5-8 hours. S3: Add sodium persulfate (Na2S2O8) and hydrogen peroxide (H2O2) to the remaining filtrate after separating ferrous sulfate crystals in S2. Perform an oxidation reaction at a stirring speed of 300-700 rpm for 0.5-2 hours at a temperature of 40-60℃. The molar ratio of sodium persulfate to lithium iron phosphate is (0.5-1):2; the molar ratio of hydrogen peroxide to lithium iron phosphate is (0.1-0.4):1. Adjust the pH of the solution to 2.0-3.0 and allow it to age for 2-8 hours. Separate and wash to obtain ferric phosphate precipitate (FePO4·yH2O). S4: Add 0.5~2.0 mol / L sodium hydroxide solution to the remaining filtrate after separating the iron phosphate precipitate in S3 to adjust the pH value to 10.5~11.5, and then age for 2~8 hours. Separate, wash and dry to obtain lithium phosphate (Li3PO4). S5: Heat the remaining filtrate after separating lithium phosphate in S4 to 60-80℃; while stirring continuously, slowly add Na2CO3 solution until the pH value rises to 7.5-8.5, then age for 1-4 hours to obtain lithium carbonate precipitate (Li2CO3); add sulfuric acid to the remaining filtrate after separating the lithium carbonate precipitate and adjust the pH to 7, then stir at 50-60℃ and 200-250 rpm to slowly evaporate and crystallize to obtain anhydrous sodium sulfate (Na2SO4). S6: The iron phosphate precipitate (FePO4·yH2O) obtained in S3 is pre-sintered, mixed with sodium source, phosphorus source and carbon source, and sintered to prepare high-performance sodium-ion battery cathode material phosphate; the FeSO4·xH2O obtained in S2 is pre-sintered, mixed with anhydrous sodium sulfate obtained in S5, and a carbon source is added. After sintering, high-performance sodium-ion battery cathode material sulfate is obtained.

[0041] As a further embodiment, the temperature for pre-sintering the iron phosphate precipitate (FePO4·yH2O) in S6 is 200~350℃, and the time is 2~10 hours.

[0042] As a further embodiment, the preparation method of the high-performance sodium-ion battery cathode material phosphate in S6 is as follows: the iron phosphate precipitate (FePO4·yH2O) obtained in S3 is pre-sintered at a temperature of 200~350℃ for 2~10 hours, mixed with sodium source, phosphorus source and carbon source, and sintered to obtain the high-performance sodium-ion battery cathode material phosphate; wherein the molar ratio of iron atoms, sodium atoms and phosphorus atoms is (2~3.5):(3~4.5):(3~4.5), and the input mass of carbon source is 5%~20% of the high-performance sodium-ion battery cathode material phosphate without carbon elements.

[0043] As a further preferred embodiment, the preparation method of the high-performance sodium-ion battery cathode material phosphate in S6 is as follows: the iron phosphate precipitate (FePO4·yH2O) obtained in S3 is pre-sintered at a temperature of 200~350℃ for 2~10 hours, mixed with sodium source, phosphorus source and carbon source, and then sand-milled, spray-dried and sintered to obtain the high-performance sodium-ion battery cathode material phosphate; wherein the molar ratio of iron atoms, sodium atoms and phosphorus atoms is (2~3.5):(3~4.5):(3~4.5), and the input mass of carbon source is 5%~20% of the high-performance sodium-ion battery cathode material phosphate without carbon elements.

[0044] As a further preferred embodiment, the milling speed is 1500~2500 rpm / min and the time is 20~120min; the spray drying inlet temperature is 200~250℃, the outlet temperature is 90~110℃, and the feed rate is 5~50 mL / min.

[0045] This invention further defines the conditions for raw material milling and spray drying in the preparation method of high-performance sodium-ion battery cathode material phosphate. Milling serves to break down and uniformly disperse particles. Through the high-frequency impact and shear force of the grinding media (such as zirconium oxide beads) in the mill, the raw material particles are further refined and agglomerated, ensuring thorough mixing and forming a uniform slurry. The finer particles after milling have a larger specific surface area, resulting in more complete element diffusion and more uniform sintering during subsequent high-temperature solid-state reactions, thereby optimizing the material's initial efficiency and cycle life, among other electrochemical properties. This invention also allows for precise control of particle size distribution and morphology by adjusting the spray drying parameters, forming a micron-nano secondary structure and improving tap density and electrode performance. During spray drying, the solute precipitates uniformly as the droplets evaporate, reducing local enrichment or loss of material components and ensuring accurate stoichiometry. These further limitations enhance the electrochemical performance of the obtained high-performance sodium-ion battery cathode material phosphate.

[0046] As a further preferred embodiment, the sintering process in the preparation of the high-performance sodium-ion battery cathode material phosphate in S6 includes a first sintering pretreatment and a second sintering treatment; the first sintering pretreatment time is selected from 2 to 8 h; the temperature is selected from 300 to 400 ℃; and the heating rate is selected from 0.5 to 5 ℃ / min; the second sintering treatment time is selected from 6 to 24 h; the temperature is selected from 450 to 650 ℃; and the heating rate is selected from 0.5 to 5 ℃ / min.

[0047] This invention effectively removes residual organic matter and moisture through a first sintering pretreatment, avoiding damage to the material's crystal structure caused by direct high-temperature sintering. A subsequent second sintering treatment further promotes crystal growth and structural stabilization. Precise control of the temperature and time of both sintering processes ensures optimal electrochemical performance of the mixed phosphate cathode material. Furthermore, the introduction of hydrogen and inert gases during sintering not only reduces transition metals but also prevents side reactions, reducing impurity phase formation and thus improving the material's conductivity and electrochemical activity. Ultimately, these refined process steps facilitate the preparation of high-purity mixed phosphate cathode materials with high discharge specific capacity and capacity retention, meeting the demands of high-performance batteries.

[0048] As a further embodiment, the preparation method of the high-performance sodium-ion battery cathode material sulfate is as follows: FeSO4·xH2O obtained in S2 is pre-calcined to obtain anhydrous ferrous sulfate, which is then mixed with anhydrous sodium sulfate obtained in S5. A carbon source is added, and the mixture is ball-milled and sintered to obtain the high-performance sodium-ion battery cathode material sulfate. The pre-sintering temperature of FeSO4·xH2O is 150~350℃, and the time is 2~10 hours. The molar ratio of anhydrous sodium sulfate to anhydrous ferrous sulfate is 1:(1.3~2), and the mass of the carbon source is 3%~10% of the carbon-free high-performance sodium-ion battery cathode material sulfate.

[0049] As a further preferred embodiment, in the method for preparing the high-performance sodium-ion battery cathode material sulfate, the ball milling speed is 400~800 rpm; the ball milling time is 3~8 hours.

[0050] As a further embodiment, in the method for preparing the high-performance sodium-ion battery cathode material sulfate, the sintering time is selected from 8 to 24 h; the temperature is selected from 300 to 400 ℃; and the heating rate is selected from 0.5 to 5 ℃ / min.

[0051] As a further embodiment, the sodium source includes, but is not limited to, any one or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium citrate, sodium methoxide, sodium ethoxide, sodium pyrophosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium formate, sodium acetate, sodium nitrate, and sodium nitrite.

[0052] As a further embodiment, the phosphorus source includes, but is not limited to, any one or more of sodium phosphate, sodium pyrophosphate, iron phosphate, ammonium phosphate, ammonium hydrogen phosphate, and ammonium dihydrogen phosphate.

[0053] As a further embodiment, the carbon source includes, but is not limited to, any one or more of graphene, carbon nanotubes, carbon black, citric acid, glucose, fructose, sucrose, petroleum, paraffin wax, tar, and asphalt.

[0054] As a further preferred embodiment, the pre-sintering gas atmosphere and the sintering gas atmosphere include inert gases.

[0055] As a further preferred embodiment, the gas atmosphere comprises an inert gas and hydrogen.

[0056] As an example, the inert gas includes one or more of nitrogen, helium, neon, and argon.

[0057] As a further preferred embodiment, the mass percentage of hydrogen in the sintering gas in the gas atmosphere is 5-10%.

[0058] As a further embodiment, the method for preparing high-performance sodium-ion battery cathode materials using waste lithium iron phosphate further includes the following steps: S7: The high-performance sodium-ion battery cathode material phosphate and high-performance sodium-ion battery cathode material sulfate prepared in S6 are mixed and compounded, and then ball-milled to obtain a mixed cathode material.

[0059] As a further preferred embodiment, S7 involves mixing and compounding the high-performance sodium-ion battery cathode material phosphate and high-performance sodium-ion battery cathode material sulfate prepared in S6 at a ratio of 8:2, and then ball milling to obtain a mixed cathode material.

[0060] As a further preferred embodiment, the ball milling speed in S7 is 400~600 rpm, and the time is 3~5 h.

[0061] In a second aspect, the present invention provides a high-performance sodium-ion battery cathode material prepared by the preparation method described in the first aspect; the high-performance sodium-ion battery cathode material includes high-performance sodium-ion battery cathode material phosphate and / or high-performance sodium-ion battery cathode material sulfate.

[0062] As a further embodiment, the high-performance sodium-ion battery cathode material phosphate has the general formula Na m Fe x (PO4) p P2O7 / C, where m is selected from 3 to 4.5; x is selected from 2 to 3.5; p is selected from 1 to 2.5; and satisfies m + 2x = 3p + 4.

[0063] As a further embodiment, the high-performance sodium-ion battery cathode material, a sulfate cathode material, has the general formula Na... 2+2a Fe 2-a (SO4)3 / C, where 0≤a≤0.5.

[0064] As a further preferred embodiment, the high-performance sodium-ion battery cathode material includes high-performance sodium-ion battery cathode material phosphate and high-performance sodium-ion battery cathode material sulfate.

[0065] Thirdly, the present invention provides a positive electrode sheet; the positive electrode sheet is prepared by comprising the high-performance sodium-ion battery positive electrode material, binder, and conductive agent described in the second aspect.

[0066] As a further embodiment, the resistivity of the positive electrode is less than 300 Ω / cm.

[0067] Fourthly, the present invention provides a battery or electrical device, the battery comprising the positive electrode, negative electrode, separator, and electrolyte described in the third aspect, and the electrical device comprising a battery prepared from the positive electrode as described in the third aspect, the electrical device including but not limited to one or more of mobile phones, laptops, tablets, electric vehicles, electric bicycles, and energy storage systems.

[0068] The features and beneficial effects of this invention are as follows: This invention pioneers an integrated process combining "stepwise directional precipitation" separation with "recycling-regeneration." By precisely controlling key preparation processes such as acid dissolution, crystallization, oxidation, and pH adjustment, it achieves highly selective separation and full utilization of iron, phosphorus, and lithium elements from waste lithium iron phosphate. Its core lies in directly using the separated ferrous sulfate, iron phosphate, and end-of-life sodium sulfate mother liquor as high-quality precursors for synthesizing high-performance sodium-ion battery cathode materials (sulfates and phosphates), thus constructing a closed-loop, short-process resource conversion system under mild conditions. On the other hand, it enables high-value recycling of waste lithium iron phosphate, recovering valuable lithium elements as Li3PO4 and a small portion of Li2CO3.

[0069] The significant benefits of this method include, but are not limited to, achieving efficient recovery of lithium and above while transforming waste materials into high-value-added battery-grade products through a short-process, high-value-added synergistic recycling and material regeneration method. This avoids the generation of high-salinity wastewater at the source, resulting in both excellent environmental and economic benefits. The prepared NFPP cathode material exhibits excellent electrochemical performance. Furthermore, through the targeted design and utilization of all output products in the process, the Na2SO4-containing mother liquor generated after the main recovery step is coupled with the previously separated FeSO4 to synthesize another sodium-ion battery NFS cathode material. This ultimately achieves near-complete resource recovery of the liquid phase components, reducing or even eliminating secondary wastewater discharge at the source. The final prepared sodium-ion battery cathode material exhibits electrochemical performance comparable to or even exceeding that of chemical raw materials, providing a practical and innovative path for the resource recovery of waste lithium iron phosphate batteries and the supply of low-cost sodium battery materials. Attached Figure Description

[0070] To more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0071] Figure 1 This is a schematic flowchart of the method for preparing high-performance sodium-ion battery cathode materials using waste lithium iron phosphate according to the present invention.

[0072] Figure 2 The image shows the X-ray diffraction (XRD) pattern of the iron phosphate precursor in Example 1 of this invention.

[0073] Figure 3 The image shows the X-ray diffraction (XRD) pattern of the NFPP cathode material in Example 1 of this invention.

[0074] Figure 4 The image shows the X-ray diffraction (XRD) pattern of the NFS cathode material in Example 1 of this invention.

[0075] Figure 5 This is a scanning electron microscope (SEM) image of the NFPP cathode material of Example 1 of the present invention.

[0076] Figure 6 This is a scanning electron microscope (SEM) image of the NFS cathode material of Example 1 of the present invention.

[0077] Figure 7 This is a charge-discharge curve of the CR2032 button battery assembled with the NFPP cathode material of Embodiment 1 of the present invention during the first cycle.

[0078] Figure 8 This is a charge-discharge curve of the CR2032 button battery assembled with NFS cathode material according to Example 1 of the present invention during the first cycle. Detailed Implementation

[0079] To facilitate understanding of the present invention, a more comprehensive description of the present invention will be given below, and embodiments of the present invention will be provided, but this does not limit the scope of the present invention.

[0080] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0081] The chemical raw materials used in the following examples and comparative examples are all prior art and commercially available. The experimental apparatus and testing equipment used in the following examples and comparative examples are all conventional equipment in the art, and there are no special requirements or limitations.

[0082] As a specific example of the implementation of this invention, detailed cases are provided below: Example 1: 1578g of pretreated waste lithium iron phosphate cathode powder was mixed with 8L of 0.7 mol / L sulfuric acid solution and stirred continuously at 50°C for 3 hours. The insoluble matter was then filtered out. The insoluble matter was conductive carbon. The leachate was then evaporated and concentrated at 70°C until the solution volume was 4L. Cooling crystallization was then performed at 10°C for 6 hours. After washing and drying the precipitate, a mixed product of FeSO4·4H2O and FeSO4·7H2O was obtained. 714g of sodium persulfate (Na2S2O8) and 567g of 6% hydrogen peroxide (H2O2) were added to the remaining filtrate after separating the FeSO4 crystals, and the mixture was stirred continuously to remove the remaining Fe... 2+ All oxidized to Fe 3+ Oxidation time was 1 hour at 50℃; stirring speed was 400 rpm. After oxidation, the pH of the solution was adjusted to 2 using 0.7 mol / L H₂SO₄ or 1 mol / L NaOH, and the aging time was 4 hours. FePO₄·2H₂O precipitate was separated and washed. The filtrate after FePO₄ separation was adjusted to pH 11 with 1 mol / L NaOH to reduce the concentration of Li₂ in the solution. + With PO4 3- The precipitation conditions were met, and Li3PO4 precipitated. The aging time was 2 hours. Li3PO4 was then separated, washed, and dried. After separating Li3PO4, the mother liquor was heated to 60℃. The main components of the resulting mother liquor were Na2SO4 and Li2SO4. A 0.1 mol / L Na2CO3 solution was slowly added dropwise with continuous stirring, and the pH value was closely monitored. When the pH value rose to approximately 8.0, the addition was stopped, and stirring continued until the pH stabilized. After pH stabilization, the aging time was 2 hours to obtain lithium carbonate precipitate, which was then recovered. The remaining mother liquor was treated by adding 0.7 mol / L sulfuric acid and adjusting the pH to 7 to obtain a sodium sulfate solution. The solution was heated to 50-60℃ and slowly evaporated with stirring at 200-250 rpm. When the solution became slightly turbid, a small amount of 5g of anhydrous sodium sulfate seed crystals was added, and evaporation continued until a large amount of crystals precipitated, yielding anhydrous sodium sulfate.

[0083] The obtained FePO4·2H2O was pre-sintered under an inert atmosphere at 250℃, a heating rate of 2℃ / min, and a time of 3 hours to obtain anhydrous ferric phosphate. 452.4 g of the obtained anhydrous ferric phosphate, 212 g of sodium carbonate, 115 g of ammonium dihydrogen phosphate, and 93 g of citric acid were added to 2.5 L of deionized water and stirred at 800 rpm / min at 50℃ for 0.5 hours. The mixture was then transferred to a sand mill and milled for 1 hour at 2000 rpm / min. Following this, spray drying was performed at an inlet temperature of 220℃, an outlet temperature of 95℃, and a feed rate of 25 mL / min. After spray drying and granulation, the precursor powder was obtained. The precursor powder was placed in an argon-hydrogen mixture with a hydrogen content of 10% and subjected to a first sintering pretreatment at 350°C for 4 hours in a tube furnace at a heating rate of 2°C / min. Then, a second sintering treatment was performed at 500°C for 10 hours at a heating rate of 2°C / min to obtain the Na4Fe3(PO4)2P2O7 / C cathode material, which was collected and sealed for storage.

[0084] The obtained FeSO4·xH2O was pre-sintered under an inert atmosphere at 200℃, with a heating rate of 2℃ / min, for 5 hours to obtain anhydrous ferrous sulfate. 151.9 g of the obtained anhydrous ferrous sulfate, 92.3 g of anhydrous sodium sulfate, 6.1 g of graphene, and 6.1 g of carbon nanotubes were added to a ball mill jar and ball-milled at 700 rpm for 6 hours. The ball-milled powder was then sintered at 350℃ for 18 hours under an inert reducing atmosphere (a mixture of nitrogen and hydrogen, with hydrogen accounting for 10% by volume) at a heating rate of 2℃ / min to obtain Na. 2.364 Fe 1.818 (SO4)3 / C cathode material, collected and sealed for storage.

[0085] Example 2: The difference from Example 1 is that 361.9g of the obtained anhydrous ferric phosphate, 180.2g of sodium carbonate, 115.0g of ammonium dihydrogen phosphate, and 78.8g of citric acid were added to 2.2L of deionized water; 151.9g of the obtained anhydrous ferrous sulfate, 94.7g of anhydrous sodium sulfate, 6.1g of graphene, and 6.1g of carbon nanotubes were added to a ball mill jar for ball milling. Na was obtained. 3.4 Fe 2.4 (PO4) 1.4 P2O7 / C cathode material and Na 2.572 Fe 1.714 (SO4)3 / C cathode material.

[0086] Example 3: The difference from Example 1 is that after oxidation, the pH of the solution was adjusted to 3 using 0.7 mol / L H2SO4 or 1 mol / L NaOH, and the aging time was 4 hours. FePO4·2H2O precipitate was then separated and washed.

[0087] Example 4: The difference from Example 1 is as follows: S1: The sulfuric acid concentration was changed to 1.5 mol / L, the temperature was changed to 70℃, and the reaction time was extended to 5 hours. S2: The mixture was concentrated to 60% of its original volume by evaporation at room temperature, and then cooled to 5℃ for crystallization. S3: After oxidation, the pH was adjusted to 2.5, and the precipitation temperature was changed to room temperature (25℃). S4: The pH was adjusted to 10.8, and the aging time was shortened to 2 hours.

[0088] Example 5: The difference from Example 1 is as follows: NFPP preparation: The precursor powder was placed in an argon-hydrogen mixture with a hydrogen content of 10% and sintered in a tube furnace at a heating rate of 2℃ / min to 350℃ for a first sintering pretreatment for 4 hours; then, at a heating rate of 2℃ / min, it was sintered at 550℃ for a second sintering treatment for 10 hours. NFS preparation: Sintered at 380℃ at a rate of 2℃ / min for 12 hours.

[0089] Example 6: The difference from Example 1 is that 1578g of pretreated waste lithium iron phosphate cathode powder was mixed with 8L of sulfuric acid solution with a concentration of 0.8 mol / L and stirred continuously at a temperature of 20°C for 0.8 hours.

[0090] Example 7: The difference from Example 1 is that 1578g of pretreated waste lithium iron phosphate cathode powder was mixed with 1L of sulfuric acid solution with a concentration of 6 mol / L and stirred continuously at a temperature of 50°C for 5 hours.

[0091] Example 8: The difference from Example 1 is that 1578g of pretreated waste lithium iron phosphate cathode powder was mixed with 8L of 0.7 mol / L sulfuric acid solution and stirred continuously at 50°C for 3 hours. Then, the insoluble matter was filtered out. The insoluble matter was conductive carbon. The leachate was then evaporated and concentrated at 70°C until the solution volume was 4L. Cooling crystallization was then performed at 10°C for 6 hours. After washing and drying the precipitate, a mixed product of FeSO4·4H2O and FeSO4·7H2O was obtained. 714g of sodium persulfate (Na2S2O8) and 567g of 6% hydrogen peroxide (H2O2) were added to the remaining filtrate after separating the FeSO4 crystals, and the mixture was stirred continuously to remove the remaining Fe... 2+ All oxidized to Fe 3+Oxidation time was 1 hour at 50℃; stirring speed was 400 rpm. After oxidation, the pH of the solution was adjusted to 4 using 0.7 mol / L H₂SO₄ or 1 mol / L NaOH, and the aging time was 4 hours. FePO₄·2H₂O precipitate was separated and washed. The filtrate after FePO₄ separation was adjusted to pH 14 with 1 mol / L NaOH to reduce the concentration of Li₂ in the solution. + With PO4 3- The precipitation conditions were met, and Li3PO4 precipitated. The aging time was 2 hours. Li3PO4 was then separated, washed, and dried. After separating Li3PO4, the mother liquor was heated to 60℃. The main components of the resulting mother liquor were Na2SO4 and Li2SO4. A 0.1 mol / L Na2CO3 solution was slowly added dropwise with continuous stirring, and the pH value was closely monitored. When the pH value rose to approximately 8.0, the addition was stopped, and stirring continued until the pH stabilized. After pH stabilization, the aging time was 2 hours to obtain lithium carbonate precipitate, which was then recovered. The remaining mother liquor was treated by adding 0.7 mol / L sulfuric acid to adjust the pH to 7.5, yielding a sodium sulfate solution. This solution was heated to 50-60℃ and slowly evaporated with stirring at 200-250 rpm. When the solution became slightly turbid, a small amount of 5g of anhydrous sodium sulfate seed crystals was added, and evaporation continued until a large amount of crystals precipitated, yielding anhydrous sodium sulfate.

[0092] Example 9: The difference from Example 1 is that 714g of sodium persulfate (Na2S2O8) was added to the remaining filtrate after separating the FeSO4 crystals, and the mixture was stirred continuously to remove the remaining Fe in the filtrate. 2+ Oxidized to Fe 3+ The oxidation time was 1 hour at 50℃, and the stirring speed was 400 rpm. After oxidation, the pH of the solution was adjusted to 2 using 0.7 mol / L H2SO4 or 1 mol / L NaOH, and the aging time was 4 hours. The FePO4·2H2O precipitate was then separated and washed.

[0093] Example 10: The difference from Example 1 is that 567g of 6% hydrogen peroxide (H2O2) was added to the remaining filtrate after separating the FeSO4 crystals, and the mixture was stirred continuously to remove the remaining Fe in the filtrate. 2+ All oxidized to Fe 3+ The oxidation time was 1 hour at 50℃, and the stirring speed was 400 rpm. After oxidation, the pH of the solution was adjusted to 2 using 0.7 mol / L H2SO4 or 1 mol / L NaOH, and the aging time was 4 hours. The FePO4·2H2O precipitate was then separated and washed.

[0094] Example 11: The specific steps and preparation method are the same as in Example 1, except that the prepared NFPP and NFS materials are mixed and compounded. The specific operation is as follows: the two dried materials are placed in a ball mill jar at an 8:2 ratio and mechanically mixed using dry ball milling. Ball milling not only ensures uniform mixing but also appropriately refines the particles and increases contact. The ball mill speed is 500 rpm, and the time is 4 hours. A mixed cathode material is obtained.

[0095] Comparative Example 1: The specific steps and preparation method are the same as in Example 1, except that the FeSO4 recrystallization step in S2 is not performed, all iron elements are collected in the form of FePO4·2H2O precipitate, and NFPP material is prepared. 1578g of pretreated waste lithium iron phosphate cathode powder was mixed with 8L of 0.7 mol / L sulfuric acid solution and stirred continuously at 50℃ for 3 hours. Then, the insoluble matter was filtered out. 1190.6g of sodium persulfate (Na₂S₂O₈) and 567g of 6% hydrogen peroxide (H₂O₂) were added to the solution and stirred continuously. The remaining Fe in the filtrate was then removed. 2+ All oxidized to Fe 3+ Oxidation time was 1 hour at 50℃, with a stirring speed of 400 rpm. After oxidation, the pH of the solution was adjusted to 2 using 0.7 mol / L H₂SO₄ or 1 mol / L NaOH, and the aging time was 4 hours. FePO₄·2H₂O precipitate was separated and washed. The filtrate after FePO₄ separation was adjusted to pH 11 with 1 mol / L NaOH to reduce the concentration of Li₂ in the solution. + With PO4 3-The precipitation conditions were met, and Li3PO4 precipitated. The aging time was 2 hours. Li3PO4 was then separated, washed, and dried. After separating Li3PO4, the mother liquor was heated to 60℃. The main components of the resulting mother liquor were Na2SO4 and Li2SO4. A 0.1 mol / L Na2CO3 solution was slowly added dropwise with continuous stirring, and the pH value was closely monitored. When the pH value rose to approximately 8.0, the addition was stopped, and stirring continued until the pH stabilized. After pH stabilization, the aging time was 2 hours to obtain lithium carbonate precipitate, which was then recovered. The remaining mother liquor was treated by adding 0.7 mol / L sulfuric acid and adjusting the pH to 7 to obtain a sodium sulfate solution. The solution was heated to 50-60℃ and slowly evaporated with stirring at 200-250 rpm. When the solution became slightly turbid, a small amount of 5g of anhydrous sodium sulfate seed crystals was added, and evaporation continued until a large amount of crystals precipitated, yielding anhydrous sodium sulfate. The obtained FePO4·2H2O was pre-sintered under an inert atmosphere at 250℃, a heating rate of 2℃ / min, and a time of 3 hours to obtain anhydrous ferric phosphate. 452.4 g of the obtained anhydrous ferric phosphate, 212 g of sodium carbonate, 115 g of ammonium dihydrogen phosphate, and 93 g of citric acid were added to 2.5 L of deionized water and stirred at 800 rpm / min at 50℃ for 0.5 hours. The mixture was then transferred to a sand mill and milled for 1 hour at 2000 rpm / min. Following this, spray drying was performed at an inlet temperature of 220℃, an outlet temperature of 95℃, and a feed rate of 25 mL / min. After spray drying and granulation, the precursor powder was obtained. The precursor powder was placed in an argon-hydrogen mixture with a hydrogen content of 10% and subjected to a first sintering pretreatment at 350°C for 4 hours in a tube furnace at a heating rate of 2°C / min. Then, a second sintering treatment was performed at 500°C for 10 hours at a heating rate of 2°C / min to obtain the Na4Fe3(PO4)2P2O7 / C cathode material, which was collected and sealed for storage.

[0096] Comparative Example 2: 1000g of the same raw material was treated using the industry-standard "acid leaching-co-precipitation-lithium precipitation" process. The raw material was completely dissolved using sulfuric acid and hydrogen peroxide, Fe... 2+ Oxidized to Fe 3+ Adjust the pH of the solution to 2.0 with NaOH, so that Fe... 3+ and PO4 3- The co-precipitate is FePO4, but at the same time a large amount of Li + The adsorbed material is carried into the precipitate. After filtration, a saturated Na₂CO₃ solution is added to the filtrate to precipitate Li₂CO₃. The preparation method of the cathode material is the same as in Example 1.

[0097] Comparative Example 3: 1000g of the same raw material was treated using a patented recycling process. The raw material and wash water were mixed at a solid-liquid ratio of 1:1 to form a slurry, which was reacted for 10 minutes to obtain slurry 1. Concentrated sulfuric acid (98% by mass) and hydrogen peroxide (30% concentration) were added to slurry 1, with the amount of hydrogen peroxide added being three times the theoretical amount. Concentrated sulfuric acid was added until the final pH reached 2.0. After reacting at 70℃ for 90 minutes, the mixture was filtered to obtain a primary leachate and a primary carbon-containing iron-phosphorus slag. The positive and negative electrode powders were mixed with the primary leachate at a solid-liquid ratio of 1:1 to form a slurry, which was reacted for 10 minutes to obtain slurry 2. Concentrated sulfuric acid (98% by mass) and hydrogen peroxide (30% concentration) were added to slurry 2, with the amount of hydrogen peroxide added being three times the theoretical amount. Concentrated sulfuric acid was added until the final pH reached 2.0. After reacting at 70℃ for 90 minutes, the mixture was filtered to obtain a secondary leachate and a secondary carbon-containing iron-phosphorus slag. Sodium hydroxide, an alkaline substance, was added to the secondary leachate to adjust the pH to 12.5. The mixture was reacted at 25°C for 30 minutes, filtered, and the purified solution was obtained. Sodium carbonate solution (concentration 220 g / L) was added to the purified solution, with the amount of sodium carbonate added being 1.3 times the theoretical amount. The mixture was reacted at 70°C for 120 minutes, filtered, the precipitate was washed, and dried to obtain the lithium carbonate product. The cathode material preparation method was the same as in Example 1.

[0098] Comparative Example 4: 1000g of the same raw material was treated using a patented recycling process. The raw material was added to water to form a slurry, and then air was introduced into the aqueous solution. The oxidative leaching conditions were controlled as follows: solid-liquid ratio of 500g / L, reaction time of 1 hour, and reaction temperature of 25℃. During the process, dilute sulfuric acid was slowly added to maintain the pH at around 5. After the reaction, the solution containing lithium and iron-phosphorus slag were obtained by filtration. The pH of the lithium solution was adjusted to 11, and solid impurities were removed by filtration. The filtrate was heated to 95℃, and a saturated Na2CO3 solution was added to react and precipitate lithium carbonate. After washing and drying, the lithium carbonate product was obtained. Iron powder was added to the water-leached slag for ball milling activation. The molar amount of iron powder added was 0.55 times the molar amount of iron in the water-leached slag, and the ball milling time was 2 hours. The product after ball milling activation was dissolved with sulfuric acid. The solid-liquid ratio of the dissolved product was controlled at 250g / L, and the molar amount of sulfuric acid used was 1.2 times the molar amount of iron in the material. The solution containing iron and phosphorus was obtained by filtration. The iron-to-phosphorus molar ratio in the solution was adjusted to 1 by adding phosphoric acid. This solution, along with hydrogen peroxide and sodium hydroxide solution, was then simultaneously added to a reaction vessel containing iron phosphate seed crystals. The mixture was stirred, and the pH of the reaction was controlled at 2.0. After stirring, aging, and filtration, the solid product was washed and calcined to obtain iron phosphate for batteries. The preparation method of the cathode material was the same as in Example 1.

[0099] Comparative Example 5: 1000g of the same raw material was treated using a patented recycling process. At T=70℃, t=45min, with a solution-to-powder ratio of H3PO4-H2O2 of 4:1, and under the conditions of 1.5mol / L H3PO4 + 1.5vol% H2O2, the sieved powder was leached once using the H3PO4-H2O2 system, yielding a primary leachate and primary filter residue. The primary filter residue was then leached a second time using the H3PO4-H2O2 system, yielding a secondary leachate and secondary filter residue. The primary and secondary leachates were mixed to obtain a mixed leachate. CH-90Na resin was used to remove impurities from the mixed leachate, resulting in a purified lithium-rich solution. This purified solution was evaporated and concentrated under stirring at 130℃ for 1.5h, and then dried to obtain battery-grade lithium dihydrogen phosphate. Concentrated acid was added to the secondary filter residue to dissolve it, and the insoluble matter was washed with twice the amount of deionized water to obtain a solution. Sodium sulfide (2% by mass) and iron powder (0.8% by mass) were added to the solution. The solution was purified using Tulsion-62MP resin. After purification, ferrous sulfate solution or phosphoric acid was added to adjust the iron-to-phosphorus ratio based on the test results. Hydrogen peroxide and ammonia were then added to oxidize the solution and adjust the pH, causing co-precipitation of iron phosphate. After co-precipitation, the solution was washed to obtain battery-grade iron phosphate. The preparation method of the cathode material was the same as in Example 1.

[0100] Comparative Example 6: NFPP and NFS materials were prepared using non-recycled industrial-grade fresh ferric phosphate, ferrous sulfate, and sodium sulfate products. The cathode material preparation method was the same as in Example 1.

[0101] Comparative Example 7: The difference from Example 1 is that 1578g of pretreated waste lithium iron phosphate cathode powder was mixed with 8L of sulfuric acid solution with a concentration of 0.3 mol / L and stirred continuously at a temperature of 50°C for 1 hour.

[0102] Comparative Example 8: The difference from Example 1 is that 1578g of pretreated waste lithium iron phosphate cathode powder was mixed with 8L of sulfuric acid solution with a concentration of 7mol / L and stirred continuously at a temperature of 50°C for 4 hours.

[0103] The following tests were performed on Examples 1-11 and Comparative Examples 1-8, with specific test methods and conditions as follows: XRD testing: The mixed phosphate cathode material prepared in the examples was analyzed using a Rigaku-D / max-2550pc X-ray powder diffractometer from Hitachi, Japan. Cu-k was used as the radiation source, with a wavelength of 1.5406 Å, a Ni filter, a tube current of 40 mA, a tube voltage of 40 kV, a scanning range of 10° to 90°, a scanning speed of 8° / min, and a step size of 0.02°. The phase identification and crystal structure information were analyzed using JADE 6.0 software. SEM testing: The microstructure of the cathode material prepared in the examples was observed using a HITACHI S-4800 scanning electron microscope with an accelerating voltage of 20 kV. Assembly of CR2032 button sodium-ion battery: The Prussian blue cathode material, acetylene black, and polyvinylidene fluoride (PVDF) prepared in the examples were mixed in a mass ratio of 8:1:1. The uniformly mixed slurry was coated on aluminum foil, dried, and cut into discs as the cathode. A sodium metal sheet was used as the anode, and Whatman glass fiber (GF / D) was used as the separator. The organic electrolyte was prepared from NaClO4, EC (ethylene carbonate), DEC (diethyl carbonate), and FEC (fluoroethylene carbonate). The concentration of NaClO4 was 1.0 mol / L, the volume ratio of EC to DEC was 1:1, and the mass fraction of FEC in the electrolyte was 5%. The CR2032 button battery was assembled in an argon glove box. Electrochemical performance testing: The assembled CR2032 button cells were tested using a Land battery tester manufactured by Wuhan Jinno Electronics Co., Ltd. The cells were assembled in the following order: negative electrode shell, sodium metal, separator, electrode plate, gasket, spring contact, and positive electrode shell. The assembled button cells were then sealed using a sealing machine. After completion, the cells were removed from the glove box for testing. Constant current charge-discharge performance testing: The assembled cells were installed on the channels of the LAND battery testing system. The test program was then set, initially with a 12-hour resting time. The NFPP test voltage was set to 2.0V~4.0V, and the NFS test voltage to 2.0V~4.5V. The reversible capacity of the cells at a current density of 0.2C was tested. Rate Cycling Performance Test: The assembled battery is installed on the channel of the LAND battery testing system. Then the test program is set. First, a resting time of 12 hours is set. The NFPP test voltage is 2.0V~4.0V and the NFS test voltage is 2.0V~4.5V. The capacity retention rate of the battery after 100 cycles at a 2C current density is tested.

[0104] The test results of Examples 1-11 and Comparative Examples 1-8 are shown in Table 1 below: Table 1

[0105] The recycling method for waste LFP materials of the present invention does not require the addition of phosphoric acid or phosphate, but only uses a low-cost solution, thus realizing the high-value utilization of waste lithium iron phosphate cathode materials.

[0106] As can be seen from the comparison of Examples 1-11 and Comparative Examples 1-8 in Table 1, the recycling method of the present invention has a high recovery rate of each element, and the prepared high-performance sodium-ion battery cathode materials (phosphate and sulfate) have good electrochemical performance. This method achieves near-complete resource recovery of liquid phase components, reducing or even eliminating secondary wastewater discharge at the source. Transforming the vast resources of waste lithium iron phosphate batteries into key cathode materials for emerging sodium-ion batteries not only solves the problem of lithium battery recycling but also provides an innovative raw material supply model for a low-cost, sustainable sodium battery industry, promoting the synergy and circulation of the lithium battery and sodium battery industry chains.

[0107] As can be seen from Examples 2-5 in Table 1, the process still proceeded smoothly after the key parameters were changed, and all products were successfully obtained. The final recoveries of lithium, iron, and phosphorus were all relatively high, slightly lower than in Example 1, but still at an excellent level. The electrochemical performance of the prepared sodium-ion cathode material was comparable to that of the product in Example 1. This indicates that the process parameter window of this invention is wide and its robustness is good.

[0108] A comparison of Examples 1 and 6-7 shows that the present invention further limits the concentration of the sulfuric acid solution used in the acid leaching reaction and conducts stirring at a higher temperature. Furthermore, the present invention further limits the heating temperature for evaporation and concentration, as well as the temperature and time for cooling and recrystallization. This further ensures the complete release of Fe, P, and Li elements into the solution in ionic form, providing a suitable ionic environment for the subsequent complete conversion and synthesis of the product. The further limited cooling and recrystallization time further allows Fe... 2+ and SO4 2- Preferential bonding and crystal precipitation are achieved. Under the above conditions, a more rational allocation and full utilization of elements are realized.

[0109] A comparison of Examples 1 and 8 shows that the present invention further limits the pH adjustment range of S3 to S5, thus ensuring the pH of PO4 in step S3. 3- The concentration of Fe 3+ Achieving the solubility product of FePO4 ensures high selectivity and completeness of the precipitation reaction, while effectively inhibiting Fe... 3+ This forms colloidal hydroxides or other impurity phases. Simultaneously, it ensures that a large number of protons from the phosphate group in S4 are removed, reducing the PO4 content in the solution. 3-The concentration of sodium sulfate increases sharply, driving the precipitation reaction to form Li3PO4. Adjusting the pH in S5 to 6.5-7.5 ensures the full formation of sodium sulfate and avoids the residue of impurities such as sodium carbonate.

[0110] A comparison of Examples 1 and 9-10 shows that the present invention further optimizes the use of sodium persulfate and hydrogen peroxide as oxidants in the oxidation reaction, and limits the amount of oxidants used. Sodium persulfate is a strong oxidant and can effectively oxidize Fe... 2+ Oxidized to Fe 3+ The key advantage lies in its relatively mild and stable oxidation reaction, and the fact that the byproduct produced by decomposition in aqueous solution is sodium sulfate, which is itself a component of the system and does not introduce new impurity ions, thus ensuring the purity of the subsequent products. H₂O₂ can react with Fe... 2+ The reaction generates hydroxyl radicals, which can rapidly initiate and significantly accelerate the entire oxidation process, especially in the early stages. The generated radicals can activate persulfate ions, promoting their decomposition to produce sulfate radicals, thus forming a dual radical oxidation system that greatly enhances oxidation efficiency and rate. Trace amounts of H₂O₂ and the resulting radicals can also oxidize and decompose any trace organic impurities in the solution (such as residues from battery binders), purifying the solution and facilitating the production of higher purity FePO₄.

[0111] The preparation process of the compounded cathode material in Example 11 has a reversible specific capacity of 114.7 mAh / g. Furthermore, due to the addition of sulfate from some high-performance sodium-ion battery cathode materials, the high-voltage sulfate can raise the discharge platform of the hybrid electrode to 3.28V (compared to 3.06V for phosphate cathode materials). Therefore, the energy density of the battery prepared with the hybrid electrode (363.1 Wh / kg) is slightly higher than that of the phosphate cathode material (353.7 Wh / kg). Because the sulfate content of the high-performance sodium-ion battery cathode material is relatively low, the cycle stability of the hybrid electrode is high, approximately equal to that of the phosphate electrode and significantly higher than that of the sulfate electrode. This invention innovatively achieves the full recycling and combined utilization of waste lithium iron phosphate cathode materials, ensuring sufficient iron elements for Fe... 2+ Ferrous sulfate is generated in the form of Fe to prepare sulfates for high-performance sodium-ion battery cathode materials, which also ensures that some iron can be generated as Fe. 3+ As a raw material for synthesizing high-performance sodium-ion battery cathode materials, phosphate has achieved synergistic improvements in high voltage performance, rate performance, and electronic conductivity through a compounding process.

[0112] As can be seen from Comparative Example 1, without FeSO4 pre-separation, the lithium recovery rate drops significantly, while the iron and phosphorus recovery rates decrease slightly. Furthermore, the prepared NFPP material exhibits poor electrochemical performance. This may be because the oxidation and precipitation processes are difficult to control, resulting in a high lithium impurity content in the precipitated FePO4. In addition, all lithium is recovered as lithium carbonate; however, since lithium phosphate has significantly lower solubility than lithium carbonate and precipitates more easily, the recovery efficiency is even lower than in Example 1. In summary, the recovery method in Comparative Example 1 does not achieve high-value utilization of all materials.

[0113] As shown in Comparative Example 2, the traditional "acid leaching-coprecipitation-lithium precipitation" wet recovery process suffers from low lithium recovery due to severe lithium entrainment during coprecipitation. The FePO4 obtained from coprecipitation has a high lithium impurity content (>1.5 wt%), making it unsuitable for direct use as a battery-grade precursor. Furthermore, this method is lengthy, generates significant waste, and requires multiple washings after coprecipitation to reduce lithium entrainment, resulting in substantial amounts of washing wastewater. The mother liquor after lithium precipitation is high-salt wastewater requiring additional treatment. Moreover, it fails to achieve high-value utilization; the product is merely a chemical raw material and is not coupled with downstream high-performance material preparation.

[0114] As can be seen from Comparative Examples 3-5, other recycling patents generally have higher recovery rates for relatively valuable lithium, but their recovery efficiency for iron and phosphorus is not as good as the solution of this invention. Furthermore, they also generate large amounts of high-salinity wastewater, requiring additional treatment, failing to achieve high-value utilization, and the products are merely chemical raw materials, not coupled with downstream high-performance material preparation.

[0115] As can be seen from Comparative Example 6, the performance of the NFPP material and the NFS material in the embodiments of the present invention is not much different from that of the materials prepared using non-recycled industrial-grade fresh iron phosphate, ferrous sulfate and sodium sulfate products. This proves that the present solution has successfully achieved high-value utilization and closed-loop resource regeneration of waste lithium iron phosphate, and has outstanding prospects for industrial application.

[0116] Figure 1 This is a schematic flowchart illustrating the implementation method of the present invention. Figure 2 It can be seen that the iron phosphate precursor recovered by the method of this invention has high purity, and the diffraction peaks in the XRD test correspond perfectly to the standard PDF card, with no extraneous peaks. Figure 2-8 It can be seen that the method of this invention not only successfully prepared the key cathode materials NFPP and NFS for emerging sodium-ion batteries, but also these two materials have good morphology and electrochemical performance.

[0117] The technical features of the embodiments described above can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification. Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make modifications, alterations, substitutions, and variations to the above embodiments within the scope of the present invention. Furthermore, without contradiction, those skilled in the art can combine and integrate different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

Claims

1. A method for preparing high-performance sodium-ion battery cathode materials using waste lithium iron phosphate, characterized in that... Includes the following steps: S1: Waste lithium iron phosphate is mixed with sulfuric acid solution and subjected to acid leaching reaction to obtain leachate and insoluble matter; wherein the molar ratio of waste lithium iron phosphate to sulfuric acid is 1:(0.5~1). S2: The leachate obtained from S1 is evaporated, concentrated, cooled, and recrystallized to obtain ferrous sulfate crystals (FeSO4·xH2O), where 1≤x≤10; S3: Add an oxidant to the remaining filtrate after separating ferrous sulfate crystals in S2 to carry out an oxidation reaction, adjust the pH value of the solution, and separate to obtain ferric phosphate precipitate (FePO4·yH2O), where 1≤y≤10; S4: Add alkali to the remaining filtrate after separating the iron phosphate precipitate in S3 to adjust the pH value, and then separate lithium phosphate (Li3PO4). S5: Add sodium carbonate (Na2CO3) to the remaining filtrate after separating lithium phosphate in S4 to obtain lithium carbonate precipitate (Li2CO3); add sulfuric acid to the remaining filtrate after separating the lithium carbonate precipitate, adjust the pH value of the solution, and evaporate and crystallize to obtain anhydrous sodium sulfate (Na2SO4). S6: The iron phosphate precipitate (FePO4·yH2O) obtained in S3 is pre-sintered, mixed with sodium source, phosphorus source and carbon source, and sintered to prepare high-performance sodium-ion battery cathode material phosphate; the FeSO4·xH2O obtained in S2 is pre-sintered, mixed with anhydrous sodium sulfate obtained in S5, and a carbon source is added. After sintering, high-performance sodium-ion battery cathode material sulfate is obtained.

2. The preparation method according to claim 1, characterized in that, S1 involves mixing waste lithium iron phosphate with a sulfuric acid solution of 0.5~5.0 mol / L at a stirring temperature of 25~80℃ and a stirring time of 1~5h to carry out an acid leaching reaction, thereby obtaining a leachate and insoluble matter, wherein the molar ratio of waste lithium iron phosphate to sulfuric acid is 1:(0.5~1). S2 involves evaporating and concentrating the leachate obtained in S1, then cooling and recrystallizing it to obtain ferrous sulfate crystals (FeSO4·xH2O); wherein the heating temperature for evaporation and concentration is 60~80℃, the temperature for cooling and recrystallization is 5~15℃, and the standing time for cooling and recrystallization is 4~12 hours. Preferably, the reaction equation for S1 is: 2LiFePO4 + H2SO4 → 2Li + + 2Fe 2+ + 2PO4 3- + SO4 2- + H2 ; Preferably, the ferrous sulfate crystals (FeSO4·xH2O) are FeSO4·4H2O and / or FeSO4·7H2O; Preferably, the leachate in S1 is evaporated to 25% to 75% of the original solution volume.

3. The preparation method according to claim 1, characterized in that, S3 is to adjust the pH of the solution to 2-5; S4 is to add alkali to adjust the pH to 9-13; and S5 is to adjust the pH of the solution to 6.5-7.

5. Preferably, the oxidant in S3 is sodium persulfate (Na2S2O8) and / or hydrogen peroxide (H2O2). More preferably, the oxidant in S3 is sodium persulfate (Na2S2O8) and hydrogen peroxide (H2O2). More preferably, the molar ratio of sodium persulfate to lithium iron phosphate is (0.5~1):2; and the molar ratio of hydrogen peroxide to lithium iron phosphate is (0.1~0.4):

1. More preferably, step S3 involves adding sodium persulfate (Na2S2O8) and hydrogen peroxide (H2O2) to the remaining filtrate after separating ferrous sulfate crystals in step S2 for an oxidation reaction, wherein the molar ratio of sodium persulfate to lithium iron phosphate is (0.5~1):2; the molar ratio of hydrogen peroxide to lithium iron phosphate is (0.1~0.4):1; the pH of the solution is adjusted to 2.0~5.0, and ferric phosphate precipitate (FePO4·yH2O) is obtained by separation and washing. Preferably, the concentration of the hydrogen peroxide solution is selected from 0.5% to 30%; Preferably, in step S3, an oxidant is added and the mixture is stirred at a speed of 100-1000 rpm to carry out an oxidation reaction. The oxidation reaction time is 0.5-2 hours, and the oxidation reaction temperature is 20-60°C. Preferably, the pH of the solution in S3 is adjusted to 2.0~3.0; Preferably, after adjusting the pH in step S3, the precipitate is further aged for 2 to 8 hours, and the precipitate comprises FePO4·2H2O; More preferably, step S3 involves adding sodium persulfate (Na2S2O8) and hydrogen peroxide (H2O2) to the remaining filtrate after separating ferrous sulfate crystals in step S2, and carrying out an oxidation reaction at a stirring speed of 100-1000 rpm for 0.5-2 hours at a temperature of 20-60°C. The molar ratio of sodium persulfate to lithium iron phosphate is (0.5-1):2, and the molar ratio of hydrogen peroxide to lithium iron phosphate is (0.1-0.4):

1. After adjusting the pH of the solution to 2.0-3.0, the solution is aged for 2-8 hours, and then ferric phosphate precipitate (FePO4·yH2O) is obtained by separation and washing.

4. The preparation method according to claim 1, characterized in that, The pH in S4 is adjusted to 10.5~11.5; Preferably, the alkali in S4 is a NaOH solution with a concentration of 0.5~2.0 mol / L; Preferably, in step S4, after adjusting the pH, the mixture is aged for 2 to 8 hours. More preferably, step S4 involves adding 0.5-2.0 mol / L sodium hydroxide solution to the remaining filtrate after separating the ferric phosphate precipitate in step S3 to adjust the pH to 10.5-11.5, followed by aging for 2-8 hours, separation, washing, and drying to obtain lithium phosphate (Li3PO4). Preferably, step S5 involves heating the remaining filtrate after separating lithium phosphate in step S4 to 60-80°C; and slowly adding Na2CO3 solution dropwise while continuously stirring until the pH value rises to 7.5-8.

5. Preferably, the concentration of the Na2CO3 solution in S5 is 0.02~1.0 mol / L; Preferably, in step S5, the pH value is raised to 7.5-8.5 and then aged for 1-4 hours; More preferably, in step S5, sulfuric acid is added to the remaining filtrate after separating the lithium carbonate precipitate, and the pH is adjusted to 7. The mixture is then stirred at a rate of 200-250 rpm at a temperature of 50-60°C to slowly evaporate and crystallize anhydrous sodium sulfate (Na2SO4). More preferably, in step S5, sulfuric acid with a concentration of 0.02~1.0 mol / L is added to the remaining filtrate after separating the lithium carbonate precipitate, and the pH is adjusted to 7. The mixture is then slowly evaporated and crystallized at a temperature of 50~60℃ and a stirring rate of 200~250 rpm to obtain anhydrous sodium sulfate (Na2SO4).

5. The preparation method according to claim 1, characterized in that, The preparation method includes the following steps: S1: Waste lithium iron phosphate is mixed with sulfuric acid solution with a concentration of 0.5~5.0 mol / L at a stirring temperature of 40~60℃ and a stirring time of 1~5h to carry out acid leaching reaction to obtain leachate and insoluble matter; wherein the molar ratio of waste lithium iron phosphate to sulfuric acid is 1:(0.5~1); S2: The leachate obtained from S1 is evaporated, concentrated, cooled, recrystallized, washed, and dried to obtain ferrous sulfate crystals (FeSO4·xH2O). The heating temperature for evaporation and concentration is 60-80°C, the temperature for cooling and recrystallization is 7-12°C, and the settling time for cooling and recrystallization is 5-8 hours. S3: Add sodium persulfate (Na2S2O8) and hydrogen peroxide (H2O2) to the remaining filtrate after separating ferrous sulfate crystals in S2. Perform an oxidation reaction at a stirring speed of 300-700 rpm for 0.5-2 hours at a temperature of 40-60℃. The molar ratio of sodium persulfate to lithium iron phosphate is (0.5-1):2; the molar ratio of hydrogen peroxide to lithium iron phosphate is (0.1-0.4):

1. Adjust the pH of the solution to 2.0-3.0 and allow it to age for 2-8 hours. Separate and wash to obtain ferric phosphate precipitate (FePO4·yH2O). S4: Add 0.5~2.0 mol / L sodium hydroxide solution to the remaining filtrate after separating the iron phosphate precipitate in S3 to adjust the pH value to 10.5~11.5, and then age for 2~8 hours. Separate, wash and dry to obtain lithium phosphate (Li3PO4). S5: Heat the remaining filtrate after separating lithium phosphate in S4 to 60-80℃; while stirring continuously, slowly add Na2CO3 solution until the pH value rises to 7.5-8.5, then age for 1-4 hours to obtain lithium carbonate precipitate (Li2CO3); add sulfuric acid to the remaining filtrate after separating the lithium carbonate precipitate and adjust the pH to 7, then stir at 50-60℃ and 200-250 rpm to slowly evaporate and crystallize to obtain anhydrous sodium sulfate (Na2SO4). S6: The iron phosphate precipitate (FePO4·yH2O) obtained in S3 is pre-sintered, mixed with sodium source, phosphorus source and carbon source, and sintered to prepare high-performance sodium-ion battery cathode material phosphate; the FeSO4·xH2O obtained in S2 is pre-sintered, mixed with anhydrous sodium sulfate obtained in S5, and a carbon source is added. After sintering, high-performance sodium-ion battery cathode material sulfate is obtained.

6. The preparation method according to claim 1, characterized in that, The pre-sintering temperature of the iron phosphate precipitate (FePO4·yH2O) in S6 is 200~350℃, and the time is 2~10 hours; Preferably, the preparation method of the high-performance sodium-ion battery cathode material phosphate in S6 is as follows: the iron phosphate precipitate (FePO4·yH2O) obtained in S3 is pre-sintered at a temperature of 200~350℃ for 2~10 hours, mixed with sodium source, phosphorus source and carbon source, and sintered to obtain the high-performance sodium-ion battery cathode material phosphate; wherein the molar ratio of iron atoms, sodium atoms and phosphorus atoms is (2~3.5):(3~4.5):(3~4.5), and the input mass of carbon source is 5%~20% of the high-performance sodium-ion battery cathode material phosphate without carbon elements; More preferably, the preparation method of the high-performance sodium-ion battery cathode material phosphate in S6 is as follows: the iron phosphate precipitate (FePO4·yH2O) obtained in S3 is pre-sintered at a temperature of 200~350℃ for 2~10 hours, mixed with sodium source, phosphorus source and carbon source, and then sand-milled, spray-dried and sintered to obtain the high-performance sodium-ion battery cathode material phosphate; wherein the molar ratio of iron atoms, sodium atoms and phosphorus atoms is (2~3.5):(3~4.5):(3~4.5), and the input mass of carbon source is 5%~20% of the high-performance sodium-ion battery cathode material phosphate without carbon elements; Preferably, the milling speed is 1500~2500 rpm / min and the time is 20~120 min; the spray drying inlet temperature is 200~250℃, the outlet temperature is 90~110℃, and the feed rate is 5~50 mL / min. Preferably, the sintering process in the preparation of the high-performance sodium-ion battery cathode material phosphate in S6 includes a first sintering pretreatment and a second sintering treatment; the first sintering pretreatment time is selected from 2 to 8 h; the temperature is selected from 300 to 400 ℃; and the heating rate is selected from 0.5 to 5 ℃ / min; the second sintering treatment time is selected from 6 to 24 h; the temperature is selected from 450 to 650 ℃; and the heating rate is selected from 0.5 to 5 ℃ / min. Preferably, the preparation method of the high-performance sodium-ion battery cathode material sulfate is as follows: FeSO4·xH2O obtained in S2 is pre-calcined to obtain anhydrous ferrous sulfate, which is then mixed with anhydrous sodium sulfate obtained in S5. A carbon source is added, and the mixture is ball-milled and sintered to obtain the high-performance sodium-ion battery cathode material sulfate. The pre-sintering temperature of FeSO4·xH2O is 150-350℃, and the time is 2-10 hours. The molar ratio of anhydrous sodium sulfate to anhydrous ferrous sulfate is 1:(1.3~2), and the mass of the carbon source is 3%~10% of the carbon-free high-performance sodium-ion battery cathode material sulfate. More preferably, in the method for preparing the high-performance sodium-ion battery cathode material sulfate, the ball milling speed is 400~800 rpm; the ball milling time is 3~8 hours. More preferably, in the method for preparing the high-performance sodium-ion battery cathode material sulfate, the sintering time is selected from 8 to 24 h; the temperature is selected from 300 to 400 ℃; and the heating rate is selected from 0.5 to 5 ℃ / min. More preferably, the method for preparing high-performance sodium-ion battery cathode materials using waste lithium iron phosphate further includes the following steps: S7: The high-performance sodium-ion battery cathode material phosphate and high-performance sodium-ion battery cathode material sulfate prepared in S6 are mixed and compounded, and then ball-milled to obtain a mixed cathode material. More preferably, step S7 involves mixing and compounding the high-performance sodium-ion battery cathode material phosphate and high-performance sodium-ion battery cathode material sulfate prepared in step S6 at a ratio of 8:2, and then ball milling the mixture to obtain a mixed cathode material. In a further preferred embodiment, the ball milling speed in S7 is 400~600 rpm, and the time is 3~5 h.

7. A sodium-ion battery cathode material prepared by the preparation method according to any one of claims 1 to 6, characterized in that, The high-performance sodium-ion battery cathode material includes high-performance sodium-ion battery cathode material phosphate and / or high-performance sodium-ion battery cathode material sulfate.

8. The sodium-ion battery cathode material according to claim 7, characterized in that, The high-performance sodium-ion battery cathode material phosphate has the general formula Na. m Fe z (PO4) p P2O7 / C, where m is selected from 3 to 4.5; z is selected from 2 to 3.5; p is selected from 1 to 2.5; and satisfies m + 2x = 3p + 4; The high-performance sodium-ion battery cathode material, a sulfate cathode material, has the general formula Na... 2+2a Fe 2-a (SO4)3 / C, where 0≤a≤0.5; Preferably, the high-performance sodium-ion battery cathode material includes high-performance sodium-ion battery cathode material phosphate and high-performance sodium-ion battery cathode material sulfate.

9. A positive electrode sheet, characterized in that, The positive electrode sheet is prepared by the high-performance sodium-ion battery positive electrode material as described in any one of claims 7 to 8, along with a binder and a conductive agent.

10. A battery or electrical device, characterized in that... The battery comprises the positive electrode, negative electrode, separator, and electrolyte as described in claim 9; the electrical device comprises the battery prepared from the positive electrode as described in claim 9, and the electrical device comprises one or more of the following: mobile phone, laptop computer, tablet computer, electric vehicle, electric bicycle, and energy storage system.

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