Method for regenerating lithium and iron from waste lithium iron phosphate positive electrode material

Abstract

The invention provides a method for regenerating lithium and iron from a waste lithium iron phosphate positive electrode material, and belongs to the technical field of lithium batteries. The preparation method comprises the following steps: calcining a lithium iron phosphate positive electrode material to remove impurities, modifying the surface by tannic acid, sequentially coating N-graphene quantum dots and bath flower-shaped graphene, mixing with lithium carbonate, carrying out ball milling, and calcining in inert gas and hydrogen atmosphere to prepare the regenerated lithium iron phosphate positive electrode material. The regenerated lithium iron phosphate positive electrode material prepared by the invention does not destroy the original lattice structure, improves the electrochemical performance and the multi-cycle stability of lithium iron phosphate, improves the capacity retention rate, has high regeneration recovery rate, can be directly used in electric vehicles, is energy-saving and environment-friendly, realizes resource utilization of waste materials, and has a wide application prospect. Wide application prospects are realized.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and particularly relates to a method for regenerating lithium and iron from waste lithium iron phosphate cathode materials. Background Art

[0002] In recent years, the environmental pollution problem and the problem of reasonable resource recycling of waste lithium-ion power batteries have become difficult problems that are generally concerned and urgently need to be solved at home and abroad currently and in the future. Solving this problem is not only beneficial to environmental protection, but also conducive to the recycling of resources, and has great practical significance.

[0003] For different types of lithium-ion batteries, their recycling methods are also different. Since waste lithium iron phosphate materials do not contain precious metals such as cobalt and nickel, their recycling value is relatively low. Using the existing cobalt lithium oxide recycling process for recycling has no economic benefits, and the research on the recycling process of waste lithium iron phosphate materials is still in the R & D stage. Literature reports that the recycling methods of waste LiFePO4 batteries mainly fall into two categories, one is the hydrometallurgical method, and the other is the repair and regeneration method. The hydrometallurgical process uses mechanical methods to break the metal shell of the battery, and then adopts methods such as leaching, precipitation, ion exchange, and adsorption to obtain metal compounds. The metals recovered by this method have a high purity, but a large amount of acids and alkalis are used, which not only has a high cost, but also easily causes secondary pollution. More importantly, LiFePO4 batteries do not contain precious elements such as cobalt and nickel, and the economic benefits of simply recycling a certain element are not high. Therefore, the hydrometallurgical method is extremely unsuitable for recycling lithium iron phosphate power batteries, and the repair and regeneration method has become the mainstream method for treating waste lithium iron phosphate batteries at present, with high recycling benefits and the highest resource comprehensive utilization rate.

[0004] The repair and regeneration method generally includes the following steps: First, the recovered waste lithium iron phosphate batteries are disassembled, and physical methods or chemical means are used to separate the cathode material from the electrode plate. Sodium hydroxide solution is added to remove the residual aluminum in the lithium iron phosphate material, and then heat treatment is carried out to remove the residual conductive agent and binder. Appropriate iron source, lithium source or phosphorus source compounds are added to adjust the molar ratio of iron, lithium, and phosphorus to 1:1:1. Finally, a carbon source is added, and after ball milling and calcination in an inert atmosphere, a new lithium iron phosphate cathode material is obtained. Although there are literature reports that the cathode material can be successfully repaired by simply supplementing lithium and iron elements, these tests are usually half-cell tests and the number of cycles is short. Lithium iron phosphate power batteries generally have high requirements for various technical indicators of the cathode material. It is difficult to simply repair and regenerate it and reuse it for power batteries. Therefore, it is particularly important to develop new recycling and reuse technologies for waste lithium iron phosphate cathode materials of lithium-ion batteries, which can not only save resources, reduce costs, but also protect the environment. Summary of the Invention

[0005] The object of the present invention is to propose a method for regenerating lithium and iron from waste lithium iron phosphate cathode materials, without destroying the original lattice structure, improving the electrochemical performance and the stability of multiple cycles of lithium iron phosphate, increasing the capacity retention rate, having a high regeneration recovery rate, the prepared regenerated material having excellent performance, which can be directly used in electric vehicles, being energy-saving and environment-friendly, recycling waste materials, and having broad application prospects.

[0006] The technical solution of the present invention is realized as follows: The present invention provides a method for regenerating lithium and iron from waste lithium iron phosphate cathode materials. After calcining the lithium iron phosphate cathode materials to remove impurities, modifying the surface with tannic acid, sequentially coating N-graphene quantum dots and bath flower-like graphene, mixing with lithium carbonate, ball milling, and calcining in an inert gas and hydrogen atmosphere, a regenerated lithium iron phosphate cathode material is prepared.

[0007] As a further improvement of the present invention, it includes the following steps: S1. Crushing the separated waste lithium iron phosphate cathode materials, calcining to remove impurities, and ball milling to obtain a pretreated material; S2. Adding the pretreated material into water, adding tannic acid and a catalyst, heating and stirring for reaction, centrifuging, washing, and drying to obtain a modified pretreated material; S3. Adding the modified pretreated material into water, adding citric acid and urea, performing a hydrothermal reaction, centrifuging, washing, and drying to obtain an N-graphene quantum dot-coated modified pretreated material; S4. Adding the N-graphene quantum dot-coated modified pretreated material into a graphene oxide aqueous dispersion, ultrasonically dispersing evenly, spray drying, and reducing with hydrazine hydrate vapor to obtain a bath flower-like graphene / N-graphene quantum dot-coated modified pretreated material; S5. Mixing the bath flower-like graphene / N-graphene quantum dot-coated modified pretreated material with lithium carbonate, ball milling, and heating and calcining in an inert gas and hydrogen atmosphere, and heating and calcining to obtain a regenerated lithium iron phosphate cathode material.

[0008] As a further improvement of the present invention, in step S1, the temperature for calcining to remove impurities is 600 - 700 °C, the time is 20 - 40 min, and the time for ball milling is 1 - 2 h.

[0009] As a further improvement of the present invention, in step S2, the mass ratio of the pretreated material, tannic acid, and the catalyst is 10:2 - 3:0.1 - 0.2, the temperature for heating and stirring the reaction is 45 - 55 °C, the time is 2 - 3 h, and the catalyst is a Tris-HCl solution with pH = 8.5 - 9.5.

[0010] As a further improvement of the present invention, in step S3, the mass ratio of the modified pretreated material, citric acid, and urea is 10:3 - 4:0.5 - 1, the temperature for the hydrothermal reaction is 180 - 220 °C, and the time is 20 - 24 h.

[0011] As a further improvement of the present invention, in step S4, the solid-liquid ratio of the N-graphene quantum dot-coated modified pretreatment material to the graphene oxide aqueous dispersion is 1:10 - 20 g / mL, the concentration of the graphene oxide aqueous dispersion is 0.8 - 1.2 mg / mL, and the time for hydrazine hydrate vapor reduction is 7 - 10 h.

[0012] As a further improvement of the present invention, in step S5, the mass ratio of the bath flower-like graphene / N-graphene quantum dot-coated modified pretreatment material to lithium carbonate is 10:2 - 3, the ventilation rate of hydrogen in the inert gas and hydrogen atmosphere is 10 - 20 mL / min, the temperature for heating and calcining is 500 - 600 °C, the time is 1 - 2 h, the temperature for heating-up calcining is 650 - 750 °C, the time is 1 - 2 h, and the time for ball milling is 1 - 2 h.

[0013] As a further improvement of the present invention, it specifically includes the following steps: S1. Crushing the separated waste lithium iron phosphate cathode material, calcining at 600 - 700 °C for impurity removal for 20 - 40 min, and ball milling for 1 - 2 h to obtain a pretreatment material; S2. Adding 10 parts by weight of the pretreatment material to water, adding 2 - 3 parts by weight of tannic acid and 0.1 - 0.2 parts by weight of a catalyst, heating to 45 - 55 °C, stirring and reacting for 2 - 3 h, centrifuging, washing, and drying to obtain a modified pretreatment material; S3. Adding 10 parts by weight of the modified pretreatment material to water, adding 3 - 4 parts by weight of citric acid and 0.5 - 1 part by weight of urea, performing a hydrothermal reaction at 180 - 220 °C for 20 - 24 h, centrifuging, washing, and drying to obtain an N-graphene quantum dot-coated modified pretreatment material; S4. Adding the N-graphene quantum dot-coated modified pretreatment material to a 0.8 - 1.2 mg / mL graphene oxide aqueous dispersion, with the solid-liquid ratio of the N-graphene quantum dot-coated modified pretreatment material to the graphene oxide aqueous dispersion being 1:10 - 20 g / mL, ultrasonically dispersing uniformly, spray drying, and performing hydrazine hydrate vapor reduction for 7 - 10 h to obtain a bath flower-like graphene / N-graphene quantum dot-coated modified pretreatment material; S5. Mixing 10 parts by weight of the bath flower-like graphene / N-graphene quantum dot-coated modified pretreatment material with 2 - 3 parts by weight of lithium carbonate, ball milling for 1 - 2 h, in an inert gas and hydrogen atmosphere, with the ventilation rate of hydrogen being 10 - 20 mL / min, heating to 500 - 600 °C, calcining for 1 - 2 h, raising the temperature to 650 - 750 °C, and calcining for 1 - 2 h to obtain a regenerated lithium iron phosphate cathode material.

[0014] The present invention further protects a regenerated lithium iron phosphate cathode material prepared by the above method.

[0015] The present invention further protects the application of the above regenerated lithium iron phosphate cathode material in an electric vehicle.

[0016] The present invention has the following beneficial effects: The cathode material of the present invention is calcined to remove impurities, decomposing the original conductive agent and binder, avoiding the influence on the discharge capacity of the regenerated lithium iron phosphate material. Further, the surface is coated with tannic acid modification. On the one hand, it improves the adsorption and fixation of subsequent graphene quantum dots. On the other hand, it shortens the diffusion distance of subsequent Li + and retains the possibility of Li + diffusing into lithium iron phosphate nanoparticles from a three-dimensional perspective, thereby generating smaller polarization, improving the electrochemical performance of lithium iron phosphate and the stability over more than 1000 cycles, and increasing the capacity retention rate.

[0017] The present invention prepares N-doped graphene quantum dots by a solution method and adsorbs and coats them on the prepared modified pretreatment material, forming a carbon coating on the surface of the cathode material. Its conductive network is conducive to the transmission of lithium ions and electrons. At the same time, the doping of N further improves the conductivity of the material, thereby improving the electrochemical performance.

[0018] The surface of the prepared N-graphene quantum dot-coated modified pretreatment material is spray-dried to obtain a structure of flower-shaped graphene coating. The formed three-dimensional conductive network significantly reduces the electrode polarization, improves the electrochemical activity and stability, shows excellent capacity recovery, and at the same time, the flower-shaped wrinkled structure can also reduce the generation of agglomerated particles, increasing the electrochemical performance of the material.

[0019] The present invention mixes the flower-shaped graphene / N-graphene quantum dot-coated modified pretreatment material with lithium carbonate and ball-mills them. Compared with ordinary mixing, the distribution of lithium after ball-milling is more uniform, ensuring the uniform distribution of Li + inside the crystal, fundamentally eliminating the possibility of generating phase impurities such as Li3PO4 and Fe2P due to uneven distribution of lithium ions during annealing. Under a reducing gas flow of inert gas / H2, the spent cathode material is calcined and regenerated with Li2CO3, realizing the reduction of Fe 3+ so that lithium fills the vacated lithium sites, making the prepared regenerated material have excellent performance and can be directly used in electric vehicles.

[0020] The regenerated lithium iron phosphate cathode material prepared by the present invention does not damage the original lattice structure, improves the electrochemical performance of lithium iron phosphate and the stability of multiple cycles, increases the capacity retention rate, has a high regeneration recovery rate, the prepared regenerated material has excellent performance, can be directly used in electric vehicles, is energy-saving and environmentally friendly, and utilizes waste materials for resource utilization, having broad application prospects. Specific embodiments

[0021] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0022] Graphene oxide, 1000 mesh, purchased from Henan Wanying Refractory Materials Technology Co., Ltd.

[0023] Example 1

[0024] This example provides a method for regenerating lithium and iron from waste lithium iron phosphate cathode materials, including the following steps: S1. Crush the separated waste lithium iron phosphate cathode material, calcine at 600 °C for 20 min to remove impurities, and ball mill for 1 h to obtain a pretreated material; S2. Add 10 g of the pretreated material to 200 mL of water, add 2 g of tannic acid and 0.1 g of catalyst, heat to 45 °C, stir and react for 2 h, centrifuge, wash, and dry to obtain a modified pretreated material; The catalyst is a Tris-HCl solution with pH = 8.5; S3. Add 10 g of the modified pretreated material to 200 mL of water, add 3 g of citric acid and 0.5 g of urea, perform hydrothermal reaction at 180 °C for 20 h, centrifuge, wash, and dry to obtain an N-graphene quantum dot-coated modified pretreated material; S4. Add 10 g of the N-graphene quantum dot-coated modified pretreated material to 100 mL of 0.8 mg / mL graphene oxide aqueous dispersion, ultrasonically disperse at 1000 W for 10 min, spray dry, and reduce with hydrazine vapor for 7 h to obtain a bath flower-shaped graphene / N-graphene quantum dot-coated modified pretreated material; S5. Mix 10 g of the bath flower-shaped graphene / N-graphene quantum dot-coated modified pretreated material with 2 g of lithium carbonate, ball mill for 1 h, in an argon and hydrogen atmosphere, with the hydrogen flow rate of 10 mL / min, heat to 500 °C, calcine for 1 h, raise the temperature to 650 °C, and calcine for 1 h to obtain a regenerated lithium iron phosphate cathode material.

[0025] Example 2

[0026] This example provides a method for regenerating lithium and iron from waste lithium iron phosphate cathode materials, including the following steps: S1. Crush the separated waste lithium iron phosphate cathode material, calcine at 700 °C for 40 min to remove impurities, and ball mill for 2 h to obtain a pretreated material; S2. Add 10 g of the pretreated material into 200 mL of water, add 3 g of tannic acid and 0.2 g of catalyst, heat to 55 °C, stir and react for 3 h, centrifuge, wash, and dry to obtain the modified pretreated material; The catalyst is a Tris-HCl solution with a pH of 9.5; S3. Add 10 g of the modified pretreated material into 200 mL of water, add 4 g of citric acid and 1 g of urea, perform hydrothermal reaction at 220 °C for 24 h, centrifuge, wash, and dry to obtain the N-graphene quantum dot-coated modified pretreated material; S4. Add 10 g of the N-graphene quantum dot-coated modified pretreated material into 200 mL of 1.2 mg / mL graphene oxide aqueous dispersion, ultrasonically disperse at 1000 W for 10 min, spray dry, and reduce with hydrazine vapor for 10 h to obtain the flower-shaped graphene / N-graphene quantum dot-coated modified pretreated material; S5. Mix 10 g of the flower-shaped graphene / N-graphene quantum dot-coated modified pretreated material with 3 g of lithium carbonate, ball mill for 2 h, in an argon and hydrogen atmosphere with a hydrogen flow rate of 20 mL / min, heat to 600 °C, calcine for 2 h, raise the temperature to 750 °C, and calcine for 2 h to obtain the regenerated lithium iron phosphate cathode material.

[0027] Example 3

[0028] This example provides a method for regenerating lithium and iron from waste lithium iron phosphate cathode materials, including the following steps: S1. Crush the separated waste lithium iron phosphate cathode material, calcine at 650 °C for 30 min to remove impurities, and ball mill for 1.5 h to obtain the pretreated material; S2. Add 10 g of the pretreated material into 200 mL of water, add 2.5 g of tannic acid and 0.15 g of catalyst, heat to 50 °C, stir and react for 2.5 h, centrifuge, wash, and dry to obtain the modified pretreated material; The catalyst is a Tris-HCl solution with a pH of 9; S3. Add 10 g of the modified pretreated material into 200 mL of water, add 3.5 g of citric acid and 0.7 g of urea, perform hydrothermal reaction at 200 °C for 22 h, centrifuge, wash, and dry to obtain the N-graphene quantum dot-coated modified pretreated material; S4. Add 10 g of the N-graphene quantum dot-coated modified pretreated material into 150 mL of 1 mg / mL graphene oxide aqueous dispersion, ultrasonically disperse at 1000 W for 10 min, spray dry, and reduce with hydrazine vapor for 8 h to obtain the flower-shaped graphene / N-graphene quantum dot-coated modified pretreated material; S5. Mix 10 g of the bath flower-like graphene / N-graphene quantum dot-coated modified pretreatment material with 2.5 g of lithium carbonate, ball mill for 1.5 h, in an argon and hydrogen atmosphere, with the hydrogen ventilation rate being 15 mL / min, heat to 550 °C, calcine for 1.5 h, raise the temperature to 700 °C, and calcine for 1.5 h to obtain the regenerated lithium iron phosphate cathode material.

[0029] Comparative Example 1 Compared with Example 3, the difference lies in that step S2 was not carried out.

[0030] Specifically as follows: S1. Crush the separated waste lithium iron phosphate cathode material, calcine for impurity removal at 650 °C for 30 min, and ball mill for 1.5 h to obtain the pretreatment material; S2. Add 10 g of the pretreatment material to 200 mL of water, add 3.5 g of citric acid and 0.7 g of urea, carry out a hydrothermal reaction at 200 °C for 22 h, centrifuge, wash, and dry to obtain the N-graphene quantum dot-coated pretreatment material; S3. Add 10 g of the N-graphene quantum dot-coated pretreatment material to 150 mL of a 1 mg / mL graphene oxide aqueous dispersion, ultrasonically disperse at 1000 W for 10 min, spray dry, and reduce with hydrazine vapor for 8 h to obtain the bath flower-like graphene / N-graphene quantum dot-coated pretreatment material; S4. Mix 10 g of the bath flower-like graphene / N-graphene quantum dot-coated pretreatment material with 2.5 g of lithium carbonate, ball mill for 1.5 h, in an argon and hydrogen atmosphere, with the hydrogen ventilation rate being 15 mL / min, heat to 550 °C, calcine for 1.5 h, raise the temperature to 700 °C, and calcine for 1.5 h to obtain the regenerated lithium iron phosphate cathode material.

[0031] Comparative Example 2 Compared with Example 3, the difference lies in that step S3 was not carried out.

[0032] Specifically as follows: S1. Crush the separated waste lithium iron phosphate cathode material, calcine for impurity removal at 650 °C for 30 min, and ball mill for 1.5 h to obtain the pretreatment material; S2. Add 10 g of the pretreatment material to 200 mL of water, add 2.5 g of tannic acid and 0.15 g of catalyst, heat to 50 °C, stir and react for 2.5 h, centrifuge, wash, and dry to obtain the modified pretreatment material; The catalyst is a Tris-HCl solution with a pH of 9; S3. Add 10 g of the modified pretreatment material to 150 mL of a 1 mg / mL graphene oxide aqueous dispersion, ultrasonically disperse at 1000 W for 10 min, spray dry, and reduce with hydrazine vapor for 8 h to obtain the bath flower-like graphene-coated modified pretreatment material; S4. Mix 10 g of the bath flower-shaped graphene-coated modified pretreatment material with 2.5 g of lithium carbonate, ball mill for 1.5 h, in an argon and hydrogen atmosphere with a hydrogen flow rate of 15 mL / min, heat to 550 °C, calcine for 1.5 h, then raise the temperature to 700 °C and calcine for 1.5 h to obtain the regenerated lithium iron phosphate cathode material.

[0033] Comparative Example 3 Compared with Example 3, the difference is that step S4 is not carried out.

[0034] Specifically as follows: S1. Crush the separated waste lithium iron phosphate cathode material, calcine at 650 °C for 30 min to remove impurities, and ball mill for 1.5 h to obtain the pretreatment material; S2. Add 10 g of the pretreatment material to 200 mL of water, add 2.5 g of tannic acid and 0.15 g of catalyst, heat to 50 °C, stir and react for 2.5 h, centrifuge, wash, and dry to obtain the modified pretreatment material; The catalyst is a Tris-HCl solution with pH = 9; S3. Add 10 g of the modified pretreatment material to 200 mL of water, add 3.5 g of citric acid and 0.7 g of urea, carry out hydrothermal reaction at 200 °C for 22 h, centrifuge, wash, and dry to obtain the N-graphene quantum dot-coated modified pretreatment material; S4. Mix 10 g of the N-graphene quantum dot-coated modified pretreatment material with 2.5 g of lithium carbonate, ball mill for 1.5 h, in an argon and hydrogen atmosphere with a hydrogen flow rate of 15 mL / min, heat to 550 °C, calcine for 1.5 h, then raise the temperature to 700 °C and calcine for 1.5 h to obtain the regenerated lithium iron phosphate cathode material.

[0035] Comparative Example 4 Compared with Example 3, the difference is that steps S3 and S4 are not carried out.

[0036] Specifically as follows: S1. Crush the separated waste lithium iron phosphate cathode material, calcine at 650 °C for 30 min to remove impurities, and ball mill for 1.5 h to obtain the pretreatment material; S2. Add 10 g of the pretreatment material to 200 mL of water, add 2.5 g of tannic acid and 0.15 g of catalyst, heat to 50 °C, stir and react for 2.5 h, centrifuge, wash, and dry to obtain the modified pretreatment material; The catalyst is a Tris-HCl solution with pH = 9; S3. Mix 10 g of the modified pretreatment material with 2.5 g of lithium carbonate, ball mill for 1.5 h, in an argon and hydrogen atmosphere, with the hydrogen ventilation rate being 15 mL / min, heat to 550 °C, calcine for 1.5 h, raise the temperature to 700 °C, and calcine for 1.5 h to obtain the regenerated lithium iron phosphate cathode material.

[0037] Comparative Example 5 Compared with Example 3, the difference lies in that steps S2, S3, and S4 are not carried out.

[0038] S1. Crush the separated waste lithium iron phosphate cathode material, calcine for impurity removal at 650 °C for 30 min, and ball mill for 1.5 h to obtain the pretreatment material; S2. Mix 10 g of the pretreatment material with 2.5 g of lithium carbonate, ball mill for 1.5 h, in an argon and hydrogen atmosphere, with the hydrogen ventilation rate being 15 mL / min, heat to 550 °C, calcine for 1.5 h, raise the temperature to 700 °C, and calcine for 1.5 h to obtain the regenerated lithium iron phosphate cathode material.

[0039] Test Example 1 For the regenerated lithium iron phosphate cathode materials prepared in Examples 1 - 3 and Comparative Examples 1 - 5, using N-methylpyrrolidone as the dispersant, SuperP as the conductive agent, and PVDF as the binder, prepare a slurry according to the ratio of regenerated lithium iron phosphate cathode material:conductive agent:binder = 90:5:5. Coat the slurry evenly on the aluminum foil and dry it to make the positive electrode sheet. Use a metal lithium sheet as the negative electrode to form a button cell and test it with a LAND electrochemical performance tester. The performance results are shown in Table 1.

[0040] Table 1

[0041] As can be seen from the above table, the batteries prepared from the regenerated lithium iron phosphate cathode materials prepared in Examples 1 - 3 of the present invention have better electrical properties.

[0042] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for regenerating lithium and iron from discarded lithium iron phosphate positive electrode materials, characterized in that: The following steps are involved: S1. The separated waste lithium iron phosphate cathode material is crushed, calcined to remove impurities, and ball milled to obtain a pretreated material; S2. The pretreated material is added to water, tannic acid and a catalyst are added, heated and stirred for reaction, centrifuged, washed, and dried to obtain a modified pretreated material; S3. The modified pretreated material is added to water, citric acid and urea are added, hydrothermally reacted, centrifuged, washed, and dried to obtain an N-graphene quantum dot-coated modified pretreated material; S4. The N-graphene quantum dot coated modified pretreatment material is added to the graphene oxide aqueous dispersion, ultrasonically dispersed uniformly, spray dried, and reduced with hydrazine hydrate vapor to obtain a bath of flower-like graphene / N-graphene quantum dot coated modified pretreatment material; S5. The bath flower-like graphene / N-graphene quantum dot coated modified pre-treated material is mixed with lithium carbonate, ball-milled, heated and calcined in an inert gas and hydrogen atmosphere, and calcined at a high temperature to obtain a regenerated lithium iron phosphate positive electrode material.

2. The method for regenerating lithium and iron from discarded lithium iron phosphate positive electrode material according to claim 1, characterized in that: The calcination and impurity removal temperature in step S1 is 600-700° C., the time is 20-40 min, and the ball milling time is 1-2 h.

3. The method for regenerating lithium and iron from discarded lithium iron phosphate positive electrode material according to claim 1, characterized in that: The mass ratio of the pretreated material, tannic acid and catalyst in step S2 is 10:2-3:0.1-0.2, the temperature of the heating and stirring reaction is 45-55°C, the time is 2-3h, and the catalyst is a Tris-HCl solution with a pH of 8.5-9.

5.

4. The method for regenerating lithium and iron from discarded lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step S3, the mass ratio of the modified pretreated material, citric acid and urea is 10:3-4:0.5-1, the temperature of the hydrothermal reaction is 180-220° C., and the time is 20-24 hours.

5. The method for regenerating lithium and iron from waste lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step S4, the solid-to-liquid ratio of the N-graphene quantum dot coated modified pretreated material and the graphene oxide aqueous dispersion is 1:10-20 g / mL, the concentration of the graphene oxide aqueous dispersion is 0.8-1.2 mg / mL, and the time for the hydrazine hydrate vapor reduction is 7-10 h.

6. The method for regenerating lithium and iron from discarded lithium iron phosphate positive electrode material according to claim 1, characterized in that: In step S5, the mass ratio of the bath flower-shaped graphene / N-graphene quantum dot coated modified pretreated material and lithium carbonate is 10:2-3, the ventilation volume of hydrogen in the inert gas and hydrogen atmosphere is 10-20 mL / min, the heating calcination temperature is 500-600 ° C, the time is 1-2h, the temperature of the temperature-raising calcination is 650-750 ° C, the time is 1-2h, and the ball milling time is 1-2h.

7. The method for regenerating lithium and iron from waste lithium iron phosphate positive electrode material according to claim 1, characterized in that: The specific steps include: S1. The separated waste lithium iron phosphate cathode material was crushed, calcined at 600-700 ° C for 20-40 min to remove impurities, and ball milled for 1-2 h to obtain a pretreated material; S2. Add 10 parts by weight of the pretreated material to water, add 2-3 parts by weight of tannic acid and 0.1-0.2 parts by weight of a catalyst, heat to 45-55°C, stir and react for 2-3h, centrifuge, wash, and dry to obtain a modified pretreated material; S3. Add 10 parts by weight of the modified pretreated material to water, add 3-4 parts by weight of citric acid and 0.5-1 parts by weight of urea, hydrothermally react at 180-220°C for 20-24h, centrifuge, wash, and dry to obtain an N-graphene quantum dot-coated modified pretreated material; S4. The N-graphene quantum dot coated modified pretreatment material is added to a 0.8-1.2 mg / mL graphene oxide aqueous dispersion, wherein the solid-liquid ratio of the N-graphene quantum dot coated modified pretreatment material to the graphene oxide aqueous dispersion is 1:10-20 g / mL, ultrasonically dispersed uniformly, spray dried, and reduced with hydrazine hydrate vapor for 7-10 hours to obtain a bath of graphene / N-graphene quantum dot coated modified pretreatment material; S5. Mix 10 parts by weight of the bath flower-like graphene / N-graphene quantum dot coated modified pretreated material with 2-3 parts by weight of lithium carbonate, ball mill for 1-2 hours, heat to 500-600°C in an inert gas and hydrogen atmosphere with a hydrogen ventilation rate of 10-20mL / min, calcine for 1-2 hours, heat to 650-750°C, calcine for 1-2 hours, and obtain a regenerated lithium iron phosphate positive electrode material.

8. A regenerated lithium iron phosphate positive electrode material obtained by the method according to any one of claims 1 to 7.

9. Use of the regenerated lithium iron phosphate positive electrode material as claimed in claim 8 in electric vehicles.

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