Green cyclic utilization-based phosphate positive electrode active material and preparation method and application thereof
By using an alkaline sodium borohydride aqueous solution to remove aluminum impurities in lithium iron phosphate batteries and combining it with a dual conductive coating design, the environmental pollution and high cost problems in the recycling of retired lithium-ion batteries have been solved, achieving efficient, low-cost, and green recycling, and improving the electrochemical performance of phosphate cathode materials.
Patent Information
- Application Number
- CN202511152076.0
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-08-18
- Publication Date
- 2025-11-21
AI Technical Summary
Current technologies for recycling retired lithium-ion batteries suffer from environmental pollution and high costs, making it difficult to achieve efficient, low-cost, and green recycling.
An alkaline sodium borohydride aqueous solution was used to remove aluminum impurities from lithium iron phosphate batteries, and the raw materials were purified through a specific compounding process. Phosphate positive electrode active materials were prepared by combining a double conductive coating layer design, avoiding the use of strong acids and alkalis and improving the electrochemical performance of the materials.
It achieves efficient recovery and utilization of active elements such as lithium, iron, and phosphorus in a green, environmentally friendly, and low-cost manner, significantly improving the electrochemical performance of cathode materials, especially ionic and electronic conductivity, and enhancing cycle stability and rate performance.
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Figure BDA0005552700590000101
Abstract
Description
Technical Field
[0001] This invention relates to the field of secondary batteries, specifically to a green and recyclable phosphate cathode active material, its preparation method, and its application. Background Technology
[0002] With the application of new energy vehicles, large-scale energy storage, and industrial and commercial energy storage, lithium-ion batteries will face large-scale retirement in five to ten years. Retired batteries contain a large amount of usable and valuable elements, but also present recycling challenges. The traditional method involves simply and crudely crushing the waste batteries and dissolving them in strong acids and alkalis to extract lithium. This process generates large amounts of wastewater, hazardous waste, and solid waste, causing environmental pollution and incurring high costs. Retired battery recycling has always been a major social problem.
[0003] How to efficiently, cost-effectively, and environmentally friendly recycle retired batteries has always been a hot research topic. How to disassemble retired batteries in a refined manner, utilize them in a short period of time through technological innovation, avoid the use of strong acids and alkalis, and achieve green recycling is a pain point in the industry. Summary of the Invention
[0004] To address the aforementioned problems, one objective of this invention is to provide a phosphate cathode active material prepared from materials in waste or retired lithium iron phosphate batteries. This method involves decomposing and recycling the cathode active material from the waste or retired lithium iron phosphate batteries, and then reconstructing the cathode material to prepare the phosphate cathode material. This method does not separate elements, is green, environmentally friendly, and low-cost, and avoids the drawbacks of traditional wet smelting lithium extraction and reuse, such as generating large amounts of wastewater and waste residue and high costs.
[0005] Because recycled lithium iron phosphate active materials are used as the main raw material, they contain a large amount of auxiliary materials such as various conductive carbons, binders, surface modifiers, and other organic matter, as well as aluminum shavings impurities in the current collector. Excessive aluminum impurities affect the crystal structure of the cathode material, particularly reducing its electrochemical performance. Using an alkaline sodium borohydride aqueous solution can effectively remove aluminum shavings, reducing their amount to a reasonable range, without causing the loss of active elements lithium, iron, and phosphorus. Furthermore, the recycled lithium iron phosphate active material contains a large amount of carbon, binders, and other auxiliary materials, and there is a risk of contamination from various foreign matter and dust during the recycling process. Directly using it as a raw material to prepare new active materials would lead to uncontrollable quality. Purifying the raw material by dissolving lithium, iron, and phosphorus in water and then filtering out impurities through a specific compounding process offers significant advantages. This specific compounding process can dissolve the vast majority of active elements, far exceeding traditional extraction processes. In addition, the composite doped conductive layer design can significantly reduce the internal resistance of phosphate-based cathode material batteries, which is of great significance for designing long-cycle, high-rate energy storage / power batteries.
[0006] To achieve the above object, the present invention adopts the following technical solutions:
[0007] The present invention provides a phosphate cathode active material based on green recycling, and the chemical general formula of the active material is LiFe x4 Nb x2 Mg x3 Ti x4 PO4 / D1D2, where 0.90 ≤ x1 < 1, 0 ≤ x2 < 0.05, 0 ≤ x3 < 0.05, 0 < x4 < 0.05; D1 and D2 are the first conductive coating layer and the second conductive coating layer respectively. The D1 conductive coating layer is an organic framework nitrogen-sulfur-boron doped composite carbon layer, and the D2 conductive coating layer is a fast ion conductor of lithium.
[0008] The D1 conductive coating layer is an organic framework nitrogen-sulfur-boron doped composite carbon layer, and the D2 conductive coating layer is a fast ion conductor of lithium; in the cathode active material, the overall mass fraction of the surface conductive coating layer in the cathode material is 1-5%, and the mass ratio of D1 to D2 is 6-10:1.
[0009] Optionally, in the cathode active material, the overall mass fraction of the surface conductive coating layer in the cathode material is 1-2%, and the mass ratio of D1 to D2 is 8-10:1.
[0010] The present invention provides a preparation method of the above-mentioned phosphate electrode active material, which includes the following steps:
[0011] S1. Put the recycled lithium iron phosphate active material into an alkaline sodium borohydride aqueous solution and stir to remove aluminum impurities. After the aluminum content is lower than 200 ppm, filter it. Put the powder obtained by washing and drying the solid phase into an aqueous solution containing a cosolvent, oxalic acid, lithium oxalate and citric acid, and carry out ball milling for 5-10 h and then filter. The lithium, iron and phosphorus elements in the filter residue are respectively lower than 1%. The filtrate is spray-dried to obtain spherical powder. The spherical powder is placed in a nitrogen atmosphere and pre-sintered at 300°C - 600°C for 1-4 h to obtain precursor powder A;
[0012] S2. Mix the nitrogen-containing organic ligand, sulfur-containing organic ligand, boric acid, and glucose evenly according to the mass fraction of 1:0.5:0.5:8, place it in a flask, reflux and heat at 200°C for 2 h under nitrogen conditions, and then cool to room temperature to obtain an organic framework-based composite carbon source precursor;
[0013] S3. According to the composition of the powder A obtained in step S1, add one or several of the carbon source precursor, Nb source compound, Mg source compound and Ti source compound correspondingly for premixing to obtain premixed powder B; add the premixed powder B, solvent and dispersant to a nano sand mill for ball milling to obtain a slurry with a primary particle size range of 100-400 nm;
[0014] S4. Spray dry and granulate the slurry obtained in step S3. Mix the granulated powder with D2 evenly, sinter in an inert atmosphere, and obtain the composite phosphate positive electrode active material LiFe after cooling. x1 Nb x2 Mg x3 Ti x4 PO4 / D1D2.
[0015] Optionally, the cosolvent in step S1 contains a macrocyclic chelate and an organophosphorus compound, wherein the macrocyclic chelate is selected from one or more of cyclodextrin, porphyrin, EDTA, and 18-crown ether-6; and the organophosphorus compound is selected from one or more of aminotrimethylenephosphonic acid, ethylenediaminetetramethylenephosphonic acid, hexamethylenediaminetetramethylenephosphonic acid, and hydroxyethylidene diphosphonic acid.
[0016] Optionally, the nitrogen-containing organic ligand in step S2 is selected from one or more of 2-methylimidazole, 2-ethylimidazole, and imidazole, and the sulfur-containing organic ligand is selected from one or more of 2,2'-dithiopyridine, 2-aminothiazole, mercaptobenzothiazole, and 1,3-diphenylthiourea.
[0017] Optionally, in step S3, the Mg source compound is selected from one or more of carbonates, oxides, nitrates, and oxalates; the Nb source compound is selected from one or more of niobium pentoxide, niobium trioxide, niobium oxide, and niobium nitride; and the Ti source compound is selected from one or more of titanium dioxide and titanium tetrachloride. The dispersant is selected from one or more of ammonium polyacrylate, polyvinylpyrrolidone, and polyethylene glycol; the solvent is deionized water; the inert atmosphere is nitrogen or argon; the mass ratio of the premixed powder to the solvent is 1:3; the mass of the dispersant accounts for 0.1-2% of the mass of the solvent; and the ball milling time is 10-15 hours.
[0018] Optionally, in step S4, the sintering temperature is 500–800°C and the sintering time is 5–15 h; D2 is selected from one or more of lithium titanate, lithium zirconate, lithium molybdate, and lithium niobate.
[0019] The present invention provides a positive electrode sheet, comprising a positive current collector and a positive active material layer coated on at least one surface of the positive current collector, wherein the positive active material layer comprises the positive active material described above.
[0020] The present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, and a separator spaced between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode described above.
[0021] The beneficial effects of the present invention include, but are not limited to:
[0022] The phosphate cathode active material provided by this invention, based on the reconstruction and utilization of waste resources, is green, environmentally friendly, and low-cost. Employing a specific compounding process, it can dissolve over 99% of the active elements such as lithium, iron, and phosphorus in recycled waste for reuse, significantly improving raw material utilization. This method is far superior to traditional extraction processes and avoids the large amounts of strong acids and alkalis used in traditional hydrometallurgical processes, greatly reducing environmental impact. By combining a double conductive coating layer with phosphate-based active materials, the electrochemical performance of the cathode material is improved, particularly the ionic and electronic conductivity, which significantly enhances the cycle stability, rate performance, and high / low temperature performance of the phosphate-based cathode material. Detailed Implementation
[0023] To make the technical solution and advantages of the present invention clearer, the present invention and its beneficial effects will be described in further detail below in conjunction with specific embodiments, but the embodiments of the present invention are not limited thereto.
[0024] Example 1
[0025] A green, recyclable phosphate cathode active material with the molecular formula LiFe 0.97 Nb 0.01 Mg 0.01 Ti 0.01 PO4 / D1D2, where D1 is a nitrogen-sulfur-boron-doped carbon layer based on an organic framework, and D2 is lithium titanate.
[0026] The preparation method of this positive electrode active material includes the following steps:
[0027] S1. The recycled lithium iron phosphate active material is added to an alkaline sodium borohydride aqueous solution and stirred to remove aluminum impurities. After the aluminum content is less than 190 ppm, it is filtered. The powder obtained after solid phase washing and drying is added to an aqueous solution containing cyclodextrin, hydroxyethylidene diphosphonic acid, oxalic acid, lithium oxalate and citric acid and ball-milled for 10 h and then filtered. The lithium, iron and phosphorus elements in the filter residue are less than 0.7% respectively. The filtrate is spray-dried to obtain spherical powder. The spherical powder is pre-sintered at 300℃ in a nitrogen atmosphere for 1 h to obtain precursor powder A.
[0028] S2. Mix 2-methylimidazole, 2,2'-dithiodipyridine, boric acid and glucose at a mass fraction of 1:0.5:0.5:8, place in a flask and reflux at 200°C for 2 hours under nitrogen, then cool to room temperature to obtain an organic framework-based composite carbon source precursor.
[0029] S3. Based on the composition of powder A obtained in step S1, one or more of the following are added: carbon source precursor, niobium carbonate, magnesium carbonate, and titanium dioxide, and premixed to obtain premixed powder B; wherein the molar ratio of Li:Fe:Nb:Mg:Ti:P is 1:0.97:0.01:0.01:0.01:1. The premixed powder B, mixed at a mass ratio of 1:3, is added to a nano-sand mill with deionized water and ammonium polyacrylate (ammonium polyacrylate is 0.8% of the mass of deionized water) and subjected to high-energy ball milling for 15 hours to obtain a slurry with a primary particle size of approximately 300 nm.
[0030] S4. Spray dry and granulate the slurry obtained in step S3. Mix the granulated powder with lithium titanate evenly, sinter at 750°C for 10 hours in a nitrogen atmosphere, and cool to obtain the recyclable positive electrode active material LiFe. 0.97 Nb 0.01 Mg 0.01 Ti 0.01 PO4 / D1D2. The conductive coating layer accounts for 2% of the total mass of the positive electrode active material, and the mass ratio of D1 to D2 is 9:1.
[0031] The above-obtained positive electrode active material is applied to positive electrode sheets and button cells. The preparation process is as follows:
[0032] Positive electrode sheet: Weigh 1.9g of the obtained positive electrode active material, add 0.05g of carbon black and 0.05g of polyvinylidene fluoride dissolved in N,N'-methylpyrrolidone, homogenize and coat it on aluminum foil to form a positive electrode sheet.
[0033] Button cell: In a glove box under an argon atmosphere, 2032 button cells were assembled using lithium metal as the counter electrode, glass fiber as the separator, and 1M / LiPF6 / PC:EMC:EC (volume ratio 1:1:1) as the electrolyte.
[0034] Example 2
[0035] A green, recyclable phosphate cathode active material with the molecular formula LiFe 0.98 Nb 0.01 Ti 0.01 PO4 / D1D2, where D1 is a nitrogen-sulfur-boron-doped carbon layer based on an organic framework, and D2 is lithium zirconate.
[0036] The preparation method of this positive electrode active material includes the following steps:
[0037] S1. The recovered lithium iron phosphate active material is added to an alkaline sodium borohydride aqueous solution and stirred to remove aluminum impurities. After the aluminum content is lower than 195 ppm, the powder obtained after solid phase washing and drying is added to an aqueous solution containing 18-crown ether-6, ethylenediaminetetramethylenephosphonic acid, oxalic acid, lithium oxalate and citric acid for ball milling for 10 h and then filtered. The lithium, iron and phosphorus elements in the filter residue are lower than 0.8% respectively. The filtrate is spray dried to obtain spherical powder. The spherical powder is pre-sintered at 400℃ in a nitrogen atmosphere for 2 h to obtain precursor powder A.
[0038] S2. Imidazole, 2-aminothiazole, boric acid and glucose were mixed evenly in a mass fraction of 1:0.5:0.5:8. The mixture was placed in a flask and refluxed at 200°C for 2 hours under nitrogen atmosphere. After cooling to room temperature, an organic framework-based composite carbon source precursor was obtained.
[0039] S3. Based on the composition of powder A obtained in step S1, one or more of the following are added for premixing: carbon source precursor, niobium carbonate, and titanium dioxide, to obtain premixed powder B; wherein the molar ratio of Li:Fe:Nb:Ti:P is 1:0.98:0.01:0.01:1. The premixed powder B, mixed at a mass ratio of 1:3, is added to deionized water and ammonium polyacrylate (ammonium polyacrylate is 1% of the mass of deionized water) and subjected to high-energy ball milling for 10 hours to obtain a slurry with a primary particle size of approximately 200 nm.
[0040] S4. The slurry is spray-dried and granulated. The granulated powder is mixed evenly with lithium zirconate and sintered at 780°C for 10 hours in a nitrogen atmosphere. After cooling, the recyclable positive electrode active material LiFe is obtained. 0.98 Nb 0.01 Ti 0.01 PO4 / D1D2. The conductive coating layer accounts for 1.7% of the total mass of the positive electrode active material, and the mass ratio of D1 to D2 is 9:1.
[0041] The above-obtained positive electrode active material is applied to positive electrode sheets and button cells. The preparation process is as follows:
[0042] Positive electrode sheet: Weigh 1.9g of the obtained positive electrode active material, add 0.05g of carbon black and 0.05g of polyvinylidene fluoride dissolved in N,N'-methylpyrrolidone, homogenize and coat it on aluminum foil to form a positive electrode sheet.
[0043] Button cell: In a glove box under an argon atmosphere, 2032 button cells were assembled using lithium metal as the counter electrode, glass fiber as the separator, and 1M / LiPF6 / PC:EMC:EC (volume ratio 1:1:1) as the electrolyte.
[0044] Example 3
[0045] A green, recyclable phosphate cathode active material with the molecular formula LiFe 0.97 Nb 0.01 Mg 0.01 Ti 0.01 PO4 / D1D2, where D1 is a nitrogen-sulfur-boron-doped carbon layer based on an organic framework, and D2 is lithium niobate.
[0046] The preparation method of this positive electrode active material includes the following steps:
[0047] S1. The recycled lithium iron phosphate active material is added to an alkaline sodium borohydride aqueous solution and stirred to remove aluminum impurities. After the aluminum content is lower than 187 ppm, the powder obtained after solid phase washing and drying is added to an aqueous solution containing EDTA, hydroxyethylidene diphosphonic acid, oxalic acid, lithium oxalate and citric acid for ball milling for 10 h and then filtered. The lithium, iron and phosphorus elements in the filter residue are lower than 0.9% respectively. The filtrate is spray dried to obtain spherical powder. The spherical powder is pre-sintered at 300℃ in a nitrogen atmosphere for 2 h to obtain precursor powder A.
[0048] S2. Mix 2-methylimidazole, 1,3-diphenylthiourea, boric acid and glucose at a mass fraction of 1:0.5:0.5:8, place in a flask and reflux at 200°C for 2 hours under nitrogen, then cool to room temperature to obtain an organic framework-based composite carbon source precursor.
[0049] S3. Based on the composition of powder A obtained in step S1, add one or more of the following: carbon source precursor, niobium carbonate, magnesium carbonate, and titanium dioxide for premixing to obtain premixed powder B; wherein the molar ratio of Li:Fe:Nb:Mg:Ti:P is 1:0.97:0.01:0.01:0.01:1. Add the premixed powder B (mixed at a mass ratio of 1:3) to deionized water and ammonium polyacrylate (ammonium polyacrylate is 1.5% of the mass of deionized water) to a nano-sand mill and perform high-energy ball milling for 10 hours to obtain a slurry with a primary particle size of approximately 400 nm.
[0050] S4. The slurry is spray-dried and granulated. The granulated powder is then mixed evenly with lithium niobate and sintered at 700°C for 12 hours in a nitrogen atmosphere. After cooling, the recyclable positive electrode active material LiFe is obtained. 0.97 Nb 0.01 Mg 0.01 Ti 0.01 PO4 / D1D2. The conductive coating layer accounts for 2% of the total mass of the positive electrode active material, and the mass ratio of D1 to D2 is 7:1.
[0051] The above-obtained positive electrode active material is applied to positive electrode sheets and button cells. The preparation process is as follows:
[0052] Positive electrode sheet: Weigh 1.9g of the obtained positive electrode active material, add 0.05g of carbon black and 0.05g of polyvinylidene fluoride dissolved in N,N'-methylpyrrolidone, homogenize and coat it on aluminum foil to form a positive electrode sheet.
[0053] Button cell: In a glove box under an argon atmosphere, 2032 button cells were assembled using lithium metal as the counter electrode, glass fiber as the separator, and 1M / LiPF6 / PC:EMC:EC (volume ratio 1:1:1) as the electrolyte.
[0054] Example 4
[0055] The difference from Example 1 is the content of each raw material in this positive electrode active material. The molar ratio of Li:Fe:Nb:Mg:Ti:P is 1:0.94:0.02:0.02:0.02:1, resulting in a sodium-ion battery positive electrode active material with the molecular formula LiFe. 0.94 Nb 0.02 Mg 0.02 Ti 0.02 PO4 / D1D2
[0056] The rest is the same as in Example 1, and will not be repeated here.
[0057] Example 5
[0058] The difference from Example 1 is the content of each raw material in this positive electrode active material, wherein the molar ratio of Li:Fe:Mg:Ti:P is 1:0.98:0.01:0.01:1, and the molecular formula of the resulting positive electrode active material for sodium-ion batteries is LiFe. 0.98 Mg 0.01 Ti 0.01 PO4 / D1D2. Unlike Example 1, the active element residue in the filter residue of step S1 is 0.7%, and the mass ratio of D1 to D2 is 6:1.
[0059] The rest is the same as in Example 1, and will not be repeated here.
[0060] Example 6
[0061] Unlike Example 1, the active element residue in the filter residue of step S1 is 0.8%, and the ratio of D1 to D2 is 10:1.
[0062] The rest is the same as in Example 1, and will not be repeated here.
[0063] Example 7
[0064] Unlike Example 1, the active element residue in the filter residue of step S1 is 0.8%, and the ratio of D1 to D2 is 8:1.
[0065] The rest is the same as in Example 1, and will not be repeated here.
[0066] Comparative Example 1
[0067] Unlike Example 1, no co-solvent is added in step S1.
[0068] The rest is the same as in Example 1, and will not be repeated here.
[0069] Comparative Example 2
[0070] Unlike Example 1, the coating layer does not contain D2.
[0071] The rest is the same as in Example 1, and will not be repeated here.
[0072] Comparative Example 3
[0073] Unlike Example 1, the coating layer does not contain D1.
[0074] The rest is the same as in Example 1, and will not be repeated here.
[0075] Comparative Example 4
[0076] Unlike Example 1, step S2 does not involve the addition of nitrogen-containing organic ligands, sulfur-containing organic ligands, or boric acid.
[0077] The rest is the same as in Example 1, and will not be repeated here.
[0078] Comparative Example 5
[0079] Unlike Example 1, the mass ratio of D1 to D2 is 5:1.
[0080] The rest is the same as in Example 1, and will not be repeated here.
[0081] Comparative Example 5
[0082] Unlike Example 1, the mass ratio of D1 to D2 is 12:1.
[0083] The rest is the same as in Example 1, and will not be repeated here.
[0084] The button batteries obtained in Examples 1-7 and Comparative Examples 1-6 were tested for their cycle performance and rate performance in the 2.0-3.8V charge-discharge range.
[0085] Cyclic performance tests were performed on a Land battery tester with a test voltage range of 2.0-3.8V. The battery's initial discharge capacity at 0.1C, capacity retention at 10C, and capacity retention after 100 cycles at 1C were recorded.
[0086] The test results are shown in Table 1.
[0087] Table 1
[0088]
[0089] The test results from Examples 1-7 and Comparative Examples 1-6 show that the phosphate-based cathode material reconstructed from waste resources exhibits excellent electrochemical performance and has great application potential. In particular, the synergistic effect of the conductive coating layer significantly improves rate performance and cycle performance. According to Example 1 and Comparative Example 1, the presence of macrocyclic chelates and organic additives can extract most of the lithium, iron, and phosphorus active elements from the raw materials, avoiding the use of strong acids and alkalis and reducing environmental impact. Examples 1 and Comparative Examples 2, 3, and 4 demonstrate that the double coating layer can improve both the electronic and ionic conductivity of the cathode material, which is crucial for reducing battery internal resistance and improving rate performance. Compared to pure carbon layers, nitrogen, sulfur, and boron doping further enhances the conductive coating effect of the carbon layer. Furthermore, Examples 1 and Comparative Examples 5 and 6 show that the synergistic effect of D1 and D2 is also very important; too high or too low a ratio is detrimental to rate performance. An excessively high D2 ratio has a negative effect, leading to uneven distribution of carbon material on the surface of the active material, thus affecting battery capacity and rate. A low D2 ratio also limits the ionic conductivity. Therefore, based on mass fraction, a ratio of 8–10:1 for the double conductive coating on the surface of the green recyclable phosphate cathode active material is optimal, resulting in better discharge specific capacity, cycle performance, and rate performance of the battery.
[0090] Based on the disclosure and teachings of the foregoing specification, those skilled in the art can make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the specific embodiments described above, and any obvious improvements, substitutions, or modifications made by those skilled in the art based on the present invention are within the scope of protection of the present invention. Furthermore, although some specific terms are used in this specification, these terms are only for convenience of explanation and do not constitute any limitation on the present invention.
Claims
1. A green, recyclable phosphate cathode active material, characterized in that, The chemical general formula is LiFe x1 Nb x2 Mg x3 Ti x4 PO4 / D1D2, where 0.90 ≤ x1 < 1, 0 ≤ x2 < 0.05, 0 ≤ x3 < 0.05, 0 < x4 < 0.05; D1 and D2 are the first conductive coating layer and the second conductive coating layer respectively. The D1 conductive coating layer is a nitrogen, sulfur, and boron-doped composite carbon layer based on an organic framework, and the D2 conductive coating layer is a fast ion conductor of lithium.
2. The green and recyclable phosphate positive electrode active material according to claim 1, characterized in that, In the positive electrode active material, the overall mass fraction of the surface conductive coating layer is 1-5% of the positive electrode material, and the mass ratio of D1 to D2 is 6-10:
1.
3. A method for preparing a green, recyclable phosphate positive electrode active material as described in any one of claims 1-2, characterized in that, Includes the following steps: S1. The recovered lithium iron phosphate active material is added to an alkaline sodium borohydride aqueous solution and stirred to remove aluminum impurities. After the aluminum content is less than 200 ppm, it is filtered. The powder obtained after solid phase washing and drying is added to an aqueous solution containing a cosolvent, oxalic acid, lithium oxalate and citric acid. After ball milling for 5 to 10 hours, it is filtered. The lithium, iron and phosphorus elements in the filter residue are less than 1% respectively. The filtrate is spray dried to obtain spherical powder. The spherical powder is placed in a nitrogen atmosphere and pre-sintered at 300℃ to 600℃ for 1 to 4 hours to obtain precursor powder A. S2. Nitrogen-containing organic ligands, sulfur-containing organic ligands, boric acid, and glucose are mixed evenly in a mass ratio of 1:0.5:0.5:
8. The mixture is placed in a flask and refluxed at 200°C for 2 hours under nitrogen atmosphere, then cooled to room temperature to obtain an organic framework-based composite carbon source precursor. S3. Based on the composition of powder A obtained in step S1, add one or more of the following: carbon source precursor, Nb source compound, Mg source compound and Ti source compound for premixing to obtain premixed powder B; add the premixed powder B, solvent and dispersant to a nano-sand mill for ball milling to obtain a slurry with a primary particle size range of 100-400 nm. S4. Spray dry and granulate the slurry obtained in step S3. Mix the granulated powder with D2 evenly, sinter in an inert atmosphere, and obtain the composite phosphate positive electrode active material LiFe after cooling. x1 Nb x2 Mg x3 Ti x4 PO4 / D1D2.
4. The preparation method of the green and recyclable phosphate positive electrode active material according to claim 3, characterized in that, The cosolvent in step S1 contains a macrocyclic chelate and an organophosphorus compound. The macrocyclic chelate is selected from one or more of cyclodextrin, porphyrin, EDTA, and 18-crown ether-6. The organophosphorus compound is selected from one or more of aminotrimethylenephosphonic acid, ethylenediaminetetramethylenephosphonic acid, hexamethylenediaminetetramethylenephosphonic acid, and hydroxyethylidene diphosphonic acid.
5. The preparation method of the green and recyclable phosphate positive electrode active material according to claim 3, characterized in that, The nitrogen-containing organic ligand in step S2 is selected from one or more of 2-methylimidazole, 2-ethylimidazole, and imidazole, and the sulfur-containing organic ligand is selected from one or more of 2,2'-dithiodipyridine, 2-aminothiazole, mercaptobenzothiazole, and 1,3-diphenylthiourea.
6. The preparation method of the green and recyclable phosphate positive electrode active material according to claim 3, characterized in that, In step S3, the Mg source compound is selected from one or more of carbonates, oxides, nitrates, and oxalates; the Nb source compound is selected from one or more of niobium pentoxide, niobium trioxide, niobium oxide, and niobium nitride; the Ti source compound is selected from one or more of titanium dioxide and titanium tetrachloride; the dispersant is selected from one or more of ammonium polyacrylate, polyvinylpyrrolidone, and polyethylene glycol; the solvent is deionized water; the inert atmosphere is nitrogen or argon; the mass ratio of the premixed powder to the solvent is 1:3; the mass of the dispersant accounts for 0.1-2% of the mass of the solvent; and the ball milling time is 10-15 hours.
7. The preparation method of the green and recyclable phosphate positive electrode active material according to claim 3, characterized in that, The sintering temperature in step S4 is 500℃~800℃, and the sintering time is 5~15h; D2 is selected from one or more of lithium titanate, lithium zirconate, lithium molybdate, and lithium niobate.
8. A positive electrode sheet, characterized in that, The positive electrode sheet includes a positive current collector and a positive active material layer coated on at least one surface of the positive current collector, wherein the positive active material layer includes the positive active material according to any one of claims 1-2.
9. A lithium-ion battery, characterized in that, The lithium-ion battery includes a positive electrode, a negative electrode, and a separator spaced between the positive electrode and the negative electrode, wherein the positive electrode is the positive electrode as described in claim 8.