Method for synthesizing LFP precursor at room temperature and pressure using waste lithium iron phosphate materials

Abstract

The present invention relates to the field of lithium battery recycling technology, and in particular to a method for synthesizing an LFP precursor using waste lithium iron phosphate materials at room temperature and pressure. The method firstly recovers and purifies lithium, iron, and phosphorus in the positive electrode material of the lithium iron phosphate battery to a mixed solution of lithium iron phosphorus after acid leaching and removing impurity metals such as copper and aluminum, and then skips the process of preparing iron phosphate and lithium carbonate and purifying them separately, directly adjusts the ratio of the three elements of lithium iron phosphorus, and coprecipitates and synthesizes the lithium iron phosphate precursor under normal temperature and pressure conditions, and can meet the use requirements of the positive electrode material of the lithium ion battery. In the present invention, the method can realize the lithium iron phosphorus coprecipitation reaction under normal temperature and pressure conditions, and then obtain the lithium iron phosphate precursor. Therefore, compared with the existing process of preparing the lithium iron phosphate precursor under high temperature and high pressure, it has the advantages of simple process route, low energy consumption, green environmental protection, etc.; and reduces the requirements for production equipment and production environment, reduces production costs, and creates higher benefits.

Description

Technical Field

[0001] The invention relates to the technical field of lithium battery recycling, and in particular to a method for synthesizing an LFP precursor at room temperature and pressure by utilizing waste lithium iron phosphate materials. Background Art

[0002] In response to the requirements of environmental protection and the energy crisis, new energy technologies are developing rapidly. In terms of energy storage technology, lithium battery technology is developing rapidly. With the increase in sales of new energy vehicles, the installation volume of lithium batteries is also increasing rapidly. In 2021, my country's lithium battery production was 324GWh, of which lithium iron phosphate battery production reached 125.4GWh. At the same time, with the increase in the production of lithium iron phosphate batteries in my country, the demand for lithium resources is also increasing year-on-year, and my country's lithium mineral resources are scarce. Therefore, my country's dependence on foreign lithium is high. As the service life of lithium iron phosphate batteries approaches, a large number of scrapped lithium iron phosphate batteries will inevitably be generated in the next few years, and the recycling of lithium iron phosphate can effectively treat waste lithium iron phosphate batteries and achieve the purpose of resource recycling and environmental protection. Recycling waste lithium iron phosphate batteries can effectively utilize elements such as iron, phosphorus, and lithium in waste batteries to achieve resource recycling.

[0003] The existing lithium iron phosphate process mainly includes three processes: waste battery pretreatment, element extraction and finished product preparation. The pretreatment of waste lithium iron phosphate mainly includes discharge, disassembly, crushing, sorting and other steps;

[0004] The methods for recovering elements of battery powder after pretreatment mainly include solid phase method, liquid phase method and other treatment methods. The solid phase method is similar to pyrometallurgy. The lithium in the battery powder is volatilized by high-temperature calcination and then captured to achieve the purpose of lithium recovery. The liquid phase method mainly uses inorganic acid to leach lithium, iron, phosphorus and other elements from the battery powder and then conduct subsequent treatment. In comparison, the liquid phase method has the advantages of high metal recovery rate, high product purity and low energy consumption. It is the mainstream process for recycling lithium iron phosphate positive electrode materials in my country.

[0005] The existing liquid phase method for recycling lithium iron phosphate positive electrode materials mainly involves leaching lithium, iron and phosphorus, separating and extracting them to prepare products. The existing process mainly directly prepares iron phosphate from the iron and phosphorus in the solution after leaching, and removes impurities from the lithium-containing solution to prepare lithium carbonate, lithium phosphate, lithium hydroxide and other products. The process involves drying, packaging, warehousing and transportation of products such as iron phosphate and lithium carbonate, resulting in a long process and high costs. Summary of the invention

[0006] The purpose of the present invention is to provide a method for synthesizing an LFP precursor at room temperature and pressure using waste lithium iron phosphate materials. The method recovers and purifies lithium, iron and phosphorus in the positive electrode material of lithium iron phosphate batteries to a mixed solution of lithium iron phosphorus, skips the process of preparing iron phosphate and lithium carbonate and purifying them separately, directly adjusts the ratio of the three elements of lithium iron phosphorus, and co-precipitates and synthesizes a lithium iron phosphate precursor at room temperature and pressure to meet the requirements of positive electrode materials for lithium ion batteries. The method reduces the requirements for production equipment and production environment, reduces production costs, and creates higher benefits.

[0007] To this end, the present invention provides a method for synthesizing an LFP precursor at room temperature and pressure using waste lithium iron phosphate materials, the method comprising the following steps:

[0008] (1) performing acid leaching on the pretreated positive electrode powder to obtain a leaching solution A and a leaching residue;

[0009] (2) adjusting the pH value of the leachate A to 2-3 to form a precipitate, filtering to obtain a filtrate G, adding a precipitant to the filtrate G to remove metal impurities such as Cu and Al, and obtaining a solution B containing lithium ions, trivalent iron ions, and phosphate after solid-liquid separation; the precipitant includes sulfide, lithium phosphate, etc., adding sulfide to obtain a Cu precipitate; adding lithium phosphate to obtain an aluminum precipitate;

[0010] Adjusting the pH value of the leachate A to 2-3 has multiple benefits in the whole process of recovering lithium iron phosphate. First, acidic conditions help to promote the dissolution of metal elements in lithium iron phosphate. In particular, in acid leaching treatment, by using an acidic solution (such as phosphoric acid, sulfuric acid), the metal elements in the battery material can be effectively dissolved. The chemical reaction conditions under this control help to improve the leaching rate and recovery efficiency of the metal. Secondly, by adjusting the pH value of the leachate A to 2-3, the reaction of removing metal impurities such as Cu and Al in step (2) provides a basic environment, which helps the subsequent treatment process, and removes impurities through the purification step to ensure the purity of the final product. Finally, the use of acidic conditions can also optimize the efficiency and cost of the entire recovery process. For example, in step (3), the reduction efficiency of trivalent iron ions is helped under acidic conditions, thereby improving the utilization rate of iron ions. In summary, adjusting the solution to acidity not only helps to improve the leaching rate and recovery efficiency of the metal in the process of recovering lithium iron phosphate, but also optimizes the purity and quality of the product, while reducing processing costs and environmental impacts.

[0011] (3) adding an appropriate amount of iron powder and VC to the solution B to carry out an iron ion reduction reaction to obtain a solution C containing lithium ions, divalent iron ions, and phosphate;

[0012] (4) adding appropriate amounts of lithium source, iron source, and phosphorus source to the solution C to adjust the contents of lithium, iron, and phosphorus to obtain a lithium-iron-phosphorus mixed solution D with a desired ratio;

[0013] (5) preparing a base solution E containing 0.05 mol / L-0.5 mol / L lithium ions and 0.016 mol / L-0.16 mol / L dihydrogen phosphate ions;

[0014] (6) The bottom liquid E is heated to 99°C-110°C under stirring conditions. Only at this reaction temperature can a high-purity product be obtained; if the temperature is too low, the coprecipitation reaction is incomplete, which will affect the purity of the product; and if the temperature is too high, the thickness of the primary particles becomes thicker, which will affect the performance of the product. The system pressure is 0.1-0.15Mpa, and then the lithium iron phosphorus mixed solution D is added to the bottom liquid E, and the system pH is adjusted to the range of 5.0-7.0 for reaction. After the reaction is completed, the lithium iron phosphate precursor is obtained by solid-liquid separation and drying;

[0015] (7) recovering lithium phosphate by adding sodium phosphate or phosphoric acid and sodium hydroxide to the mother liquor after solid-liquid separation in step (6);

[0016] (8) adding the lithium iron phosphate precursor obtained in step (6) into water, and adding a carbon source and mixing thoroughly to obtain a suspension F, wherein the solid content of the suspension F is controlled to be 35 wt % to 45 wt %; spray drying and secondary granulation of the suspension F to obtain a spherical lithium iron phosphate semi-finished product;

[0017] (9) Sintering the spherical lithium iron phosphate semi-finished product obtained in step (8) under an inert gas atmosphere at a temperature of 700-800° C.; crushing and screening the sintered material to remove iron to obtain a nano-spherical lithium iron phosphate product. The inert gas includes nitrogen and / or helium.

[0018] As can be seen from step (4), the present invention adjusts the contents of lithium, iron and phosphorus by directly adding appropriate amounts of lithium source, iron source and phosphorus source during the reaction, thereby achieving the situation where there is no need to purify lithium salt and iron salt separately, and the lithium, iron and phosphorus mixed solution after impurities are directly allowed to participate in the reaction after adjusting the ratio of lithium, iron and phosphorus, thereby saving reaction time and greatly improving production efficiency during enterprise mass production.

[0019] It can be seen from steps (5) and (6) that the present invention uses sulfuric acid to effectively leach lithium, iron and phosphorus from the components of waste lithium iron phosphate positive electrode powder, and then adjusts the ratio of lithium, iron and phosphorus elements after impurities removal. In the lithium dihydrogen phosphate solution system formed by the prepared base liquid E, the reaction pH is adjusted to 5-7.0, and then the lithium iron phosphorus co-precipitation reaction can be achieved under normal temperature and pressure conditions of pressure 0.1-0.15Mpa and temperature 99°C-110°C to obtain a lithium iron phosphate precursor. Compared with the existing process for preparing lithium iron phosphate precursors under high temperature and high pressure, the process has the advantages of simple process route, low energy consumption, and green environmental protection.

[0020] It can be seen from steps (3) and (7) that the present invention adds an appropriate amount of iron powder and VC to solution B to carry out an iron ion reduction reaction, thereby realizing the recovery of iron ions, specifically reducing trivalent iron ions to divalent iron ions, which helps to increase the recovery rate of ferrous ions to more than 99.5%. The reduced ferrous ions are used for subsequent supplementation of iron elements, thereby reducing the subsequent input of iron elements, thereby improving the recovery efficiency; and then adding sodium phosphate or phosphoric acid and sodium hydroxide to the mother liquor after solid-liquid separation, thereby realizing the recovery of lithium phosphate, and the recovered lithium phosphate can be used to prepare the base solution E, thereby realizing recycling. Under the support of the above recovery efficiency, the process route of the present invention has a higher yield, that is, higher efficiency.

[0021] Preferably, the lithium source in step (4) is lithium phosphate, lithium carbonate, lithium hydroxide, etc.; the iron source is iron powder, ferrous sulfate, etc.; and the phosphorus source is phosphoric acid, lithium phosphate, ammonium hydrogen phosphate, etc.

[0022] Preferably, the molar ratio of lithium, iron and phosphorus in step (4) is adjusted in the range of (0.6-1.00):1:(0.6-1.00). Within this ratio condition range, the ratio of lithium iron and phosphorus in the lithium iron phosphate precursor is in the range of (0.95-1.05):(0.95-1.05):(0.95-1.05), thereby obtaining a pure phase lithium iron phosphate precursor (XRD detection can be used to indicate whether it is pure phase).

[0023] Preferably, the lithium source used to prepare the bottom liquid E in step (5) can be lithium carbonate, lithium dihydrogen phosphate, lithium sulfate, lithium hydroxide, etc., and the lithium concentration range is 0.05-0.5 mol / L; the phosphorus source used to prepare the bottom liquid E in step (5) is phosphoric acid, lithium phosphate, ammonium hydrogen phosphate, etc., and the dihydrogen phosphate concentration is 0.016-0.16 mol / L. Under the condition of the bottom liquid E within such a lithium ion and dihydrogen phosphate concentration range, a coprecipitation reaction can be carried out under normal temperature and pressure conditions to prepare a lithium iron phosphate precursor. Therefore, the bottom liquid E within the lithium ion and dihydrogen phosphate concentration range is one of the necessary conditions for realizing the reaction at normal temperature and pressure of the present invention, and this concentration range is also obtained after a large number of experimental verifications.

[0024] Preferably, in step (6), alkaline substances such as liquid caustic soda, lithium carbonate, lithium hydroxide, sodium carbonate, ammonium carbonate, ammonium bicarbonate, and ammonia water are used to control the pH range of the reaction system to be 5.0-7.0. Under such a range of pH conditions, a coprecipitation reaction can be carried out under normal temperature and pressure conditions to prepare a lithium iron phosphate precursor. Therefore, reacting under this pH range is also one of the necessary conditions for realizing the reaction under normal temperature and pressure of the present invention, and this pH range is also obtained after a large number of experimental verifications.

[0025] Preferably, the addition time of the lithium iron phosphorus mixed solution D to the bottom solution E in step (6) is 4-8h. By controlling the addition time of the lithium iron phosphorus mixed solution D to the bottom solution E, the amount of nucleation can be adjusted, which will directly affect the production efficiency. Because if the addition time is too short, the amount of nucleation will be too much, and the particles will be small, and the slurry will be viscous, which will affect the purity of the lithium iron phosphate; and if the addition time is too long, the amount of nucleation will be too little, and the particles will be too large, which will affect the performance of the lithium iron phosphate and also affect the production efficiency. Therefore, controlling the addition time to 4-8h can obtain lithium iron phosphate with high purity and excellent performance, and the overall production efficiency is also high.

[0026] Preferably, the aging time after the reaction in step (6) is 4-8h. The aging process needs to keep the temperature in the range of 99°C-110°C and stir continuously. When using a small reactor for a small test, the stirring speed is 300±50r / min, when using a medium-sized reactor for a pilot test, the stirring speed is 250±50r / min, and when using a large reactor for a test, the stirring speed is 200±50r / min. The aging time needs to be controlled at 4-8h. If the aging time is too short, the purity of lithium iron phosphate will be low, which will greatly affect the performance of lithium iron phosphate.

[0027] Preferably, the mass of the carbon source added in step (8) is 5-7% of the mass of the precursor. Adding too much or too little carbon source will lead to a decrease in the electrical performance of lithium iron phosphate, specifically a decrease in the capacity of lithium iron phosphate.

[0028] The beneficial effects of the present invention are as follows:

[0029] 1. The present invention adjusts the contents of lithium, iron and phosphorus by directly adding appropriate amounts of lithium source, iron source and phosphorus source during the reaction, thereby achieving the situation where the lithium salt and iron salt do not need to be purified separately, and the lithium, iron and phosphorus mixed solution after impurities are directly allowed to participate in the reaction after adjusting the ratio of lithium, iron and phosphorus, thereby saving reaction time and greatly improving production efficiency during enterprise mass production.

[0030] 2. The present invention uses sulfuric acid to effectively leach lithium, iron and phosphorus from waste lithium iron phosphate positive electrode powder, and then adjusts the ratio of lithium, iron and phosphorus elements after impurities removal. In the lithium dihydrogen phosphate solution system formed by the prepared base liquid E, the reaction pH is adjusted to 5-7.0, and then lithium iron phosphorus co-precipitation reaction can be achieved under normal temperature and pressure conditions of pressure 0.1-0.15Mpa and temperature 99°C-110°C to obtain a lithium iron phosphate precursor. Compared with the existing process for preparing lithium iron phosphate precursors under high temperature and high pressure, the process has the advantages of simple process route, low energy consumption, and green environmental protection.

[0031] 3. The present invention adds an appropriate amount of iron powder and VC to solution B to carry out an iron ion reduction reaction, thereby realizing the recovery of iron ions, specifically reducing trivalent iron ions to divalent iron ions, which helps to increase the recovery rate of ferrous ions to more than 99.5%; and then adds sodium phosphate or phosphoric acid and sodium hydroxide to the mother liquor after solid-liquid separation, thereby realizing the recovery of lithium phosphate, which can be recycled, and under the support of the above recovery efficiency, the process route of the present invention has a higher yield, that is, higher efficiency.

[0032] 4. In the present invention, by controlling the feeding time of adding the lithium iron phosphorus mixed solution D to the bottom solution E, the amount of nucleation can be adjusted, which will directly affect the production efficiency. Because if the feeding time is too short, the amount of nucleation will be too much, and the particles will be small, and the slurry will be viscous, which will affect the purity of the lithium iron phosphate; and if the feeding time is too long, the amount of nucleation will be too little, and the particles will be too large, which will affect the performance of the lithium iron phosphate and also affect the production efficiency. Therefore, controlling the feeding time to 4-8h can obtain high-purity, excellent-performance lithium iron phosphate, and the overall production efficiency is also high. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The present invention will be further described below in conjunction with the accompanying drawings.

[0034] Figure 1 This is a process flow chart of the present invention using waste lithium iron phosphate materials to synthesize LFP precursor at room temperature and pressure;

[0035] Figure 2 This is a scanning electron microscope image of the synthesized lithium iron phosphate precursor;

[0036] Figure 3 is the X-ray diffraction (XRD) pattern of the synthesized lithium iron phosphate precursor;

[0037] Figure 4 The data of lithium iron phosphorus content in the precursor for synthesizing lithium iron phosphate;

[0038] Figure 5 The electrical performance data of the finished product of synthetic lithium iron phosphate;

[0039] Figure 6 This is an electron microscope image of the finished product of synthetic lithium iron phosphate. DETAILED DESCRIPTION

[0040] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the present invention is further explained below in combination with more detailed steps and specific implementation methods.

[0041] The present invention provides a method for synthesizing an LFP precursor at room temperature and pressure using waste lithium iron phosphate materials, the method comprising the following steps:

[0042] a. Acid dissolution: Take the waste lithium iron phosphate positive electrode material, add acid to pickle it, obtain a suspension, filter it, and obtain a filtrate①;

[0043] b. Adjust pH: Take the filtrate obtained in step a, adjust the pH value of the filtrate to 2-3, form a precipitate, filter, and obtain filtrate ②;

[0044] c. Copper removal: add sulfide to the filtrate ② to obtain Cu precipitate, filter and obtain filtrate ③;

[0045] d. Aluminum removal: add lithium phosphate to the filtrate ③ to obtain aluminum precipitate, filter and obtain solution ④;

[0046] e. Reduction: adding iron powder or VC to the filtrate ④ to reduce the ferric ions to ferrous ions, filtering to obtain a mixed solution of lithium ions, ferrous ions and phosphate ions ⑤;

[0047] f. Adjustment of the ratio of lithium iron to phosphorus: Add a lithium source, an iron source, and a phosphorus source to solution ⑤, adjust the ratio of lithium iron to phosphorus, and filter to obtain solution ⑥;

[0048] g. Synthesis reaction: Heat the base solution with a certain lithium ion and phosphate concentration to 99-110° C. under stirring, add solution ⑥ to the base solution at a certain speed, adjust the pH to 5.0-7.0, react for 4-8 hours, age for 4-8 hours, filter, wash, and dry to obtain a lithium iron phosphate precursor;

[0049] h. Carbon coating and secondary granulation: adding the lithium iron phosphate precursor obtained in step (g) into pure water, adding glucose monohydrate, sand milling with a sand mill, and then drying and secondary granulation with a spray dryer to obtain a spherical lithium iron phosphate semi-finished product;

[0050] i. Preparation of lithium iron phosphate finished products: lithium iron phosphate is sintered in a nitrogen atmosphere using a pipeline furnace, and after sintering, the sintered materials are crushed using an air flow crushing device to obtain nano-spherical lithium iron phosphate finished products.

[0051] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0052] Example 1

[0053] (1) Acid dissolution: Weigh 300 g of the positive electrode active material of the waste lithium iron phosphate battery and add it to the sulfuric acid solution. Calculate the amount of sulfuric acid added according to the stoichiometric amount of lithium in the raw material. Control the liquid-to-solid ratio to 1:5, the leaching temperature to 60°C, and the leaching time to 180 min to obtain a suspension. Filter the above suspension.

[0054] (2) Copper removal: adjust the pH of the filtrate to 2.5, add 0.2 g of sulfide, stir for 0.5 h, obtain Cu precipitate, filter, and obtain filtrate ②.

[0055] (3) Aluminum removal: Add 31.4 g of lithium phosphate to the filtrate ② to obtain an aluminum precipitate, which is filtered to obtain a solution ③;

[0056] (4) Reduction: Add 5 g of iron powder and 0.5 g of VC to the filtrate ③ to reduce the ferric ions to ferrous ions, and filter to obtain a mixed solution of lithium ions, ferrous ions and phosphate ions ④.

[0057] (5) Solution preparation: Add 21.87 g of lithium carbonate, 148.24 g of ferrous sulfate, and 47.43 g of phosphoric acid to solution ④, adjust the ratio of lithium iron to phosphorus to 1:1:1, filter, and obtain solution ⑤.

[0058] (6) Synthesis reaction: The base liquid with a certain lithium ion and dihydrogen phosphate concentration is heated to 102°C under stirring, and solution ⑤ is added to the base liquid at a certain speed. The lithium ion concentration in the base liquid is 0.3 mol / L, and the dihydrogen phosphate ion concentration is 0.1 mol / L. At the same time, ammonia water is used to adjust the pH to 5.5. The addition time is 4 h, and the aging is 5 h. After filtering, washing, and drying, a lithium iron phosphate precursor is obtained.

[0059] (7) Carbon coating and secondary granulation: 100 g of the lithium iron phosphate precursor obtained in step (6) is added to 150 g of pure water, and then 6 g of glucose monohydrate is added, and sand milled to obtain a slurry with a solid content of 40%, which is then dried and granulated again using a spray dryer to obtain a spherical lithium iron phosphate semi-finished product.

[0060] (8) Preparation of lithium iron phosphate finished product: The crude lithium iron phosphate product is sintered in a pipeline furnace at 600-790° C. in a nitrogen atmosphere for 12 hours. After sintering, the sintered material is crushed by air flow crushing equipment to obtain a nano-spherical lithium iron phosphate finished product.

[0061] Comparative Example 1

[0062] For comparison, a lithium iron phosphate precursor was prepared in the same manner.

[0063] The operating steps of Example 1 were repeated, except that the lithium ion concentration in the lithium solution in step (6) was 0.45 mol / L, the dihydrogen phosphate ion concentration was 0.15 mol / L, and other conditions were the same to obtain a lithium iron phosphate precursor.

[0064] Comparative Example 2

[0065] The operation of Example 1 was repeated, except that ammonia water was used in step (6) to control the pH value of the reaction process to 6.5. Other conditions were the same, and a lithium iron phosphate precursor was prepared.

[0066] Comparative Example 3

[0067] The operation of Example 1 was repeated, except that the feeding time in step (6) was 6 h, and other conditions were the same to prepare a lithium iron phosphate precursor.

[0068] Comparative Example 4

[0069] The operation of Example 1 was repeated, except that the ratio of lithium iron to phosphorus was adjusted to 0.9:1:0.9 in step (5), and other conditions were the same to prepare a lithium iron phosphate precursor.

[0070] Figure 2 The figure is a scanning electron microscope image of a synthesized lithium iron phosphate precursor. As can be seen from the figure, the synthesized lithium iron phosphate precursor is a rhombus-shaped sheet structure with a sheet thickness of about 100nm, a width of about 200nm, and a length of about 400nm. The sheet-like lithium iron phosphate precursor helps to form a stable crystal structure, which makes the lithium iron phosphate battery less likely to collapse under overcharge or high temperature conditions, thereby improving the safety performance of the battery. Moreover, the battery made using the sheet-like lithium iron phosphate precursor generally has a longer cycle life. Under standard test conditions, the cycle life of a single cell can reach more than 2000 times, or even higher, which helps to extend the service life of the battery. The present invention can not only carry out the synthesis reaction at room temperature and pressure through the conditions of step (6), but also obtain a sheet-like lithium iron phosphate precursor after synthesis. Therefore, the present invention can obtain a high-quality precursor at a lower synthesis cost, and thus the present invention has a greater advantage over existing synthesis methods.

[0071] Figure 3 This is an X-ray diffraction (XRD) diagram of the synthesized lithium iron phosphate precursor. It can be seen from the figure that the synthesized sample is a pure phase lithium iron phosphate precursor.

[0072] The embodiments of the present invention are described above in conjunction with the accompanying drawings, but the present invention is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Under the enlightenment of the present invention, ordinary technicians in this field can also make many forms without departing from the scope of protection of the purpose of the present invention and the claims, which all fall within the protection of the present invention.

Claims

1. A method for synthesizing an LFP precursor using waste lithium iron phosphate materials at room temperature and pressure, characterized in that: The method comprises the following steps: (1) acid leaching the pretreated positive electrode powder to obtain a leaching solution A and leaching residue; (2) adjusting the pH value of the leachate A to 2-3 to form a precipitate, filtering to obtain a filtrate G, adding a precipitant to the filtrate G to remove metal impurities, and obtaining a solution B containing lithium ions, trivalent iron ions, and phosphate after solid-liquid separation; (3) adding an appropriate amount of iron powder and VC to the solution B to carry out an iron ion reduction reaction to obtain a solution C containing lithium ions, divalent iron ions, and phosphate; (4) adding appropriate amounts of lithium source, iron source and phosphorus source to the solution C to adjust the contents of lithium, iron and phosphorus to obtain a lithium-iron-phosphorus mixed solution D with a desired ratio; (5) Prepare a base solution E containing 0.05mol / L-0.5mol / L lithium ions and 0.016mol / L-0.16mol / L dihydrogen phosphate ions; (6) heating the base liquid E to 99°C-110°C under stirring conditions, with the system pressure at 0.1-0.15 MPa, then adding the lithium iron phosphorus mixed solution D to the base liquid E, and adjusting the system pH to be in the range of 5.0-7.0 for reaction, and after the reaction is completed, solid-liquid separation and drying are performed to obtain a lithium iron phosphate precursor; (7) recovering lithium phosphate by adding sodium phosphate or phosphoric acid and sodium hydroxide to the mother liquor after solid-liquid separation in step (6); (8) adding the lithium iron phosphate precursor obtained in step (6) into water, and adding a carbon source and mixing thoroughly to obtain a suspension F, wherein the solid content of the suspension F is controlled to be 35 wt% to 45 wt%; spray drying and secondary granulation of the suspension F to obtain a spherical lithium iron phosphate semi-finished product; (9) Sintering the spherical lithium iron phosphate semi-finished product obtained in step (8) under an inert gas atmosphere at a temperature of 700 to 800° C.; crushing and screening the sintered material to remove iron to obtain a nano-spherical lithium iron phosphate finished product.

2. The method for synthesizing LFP precursor using waste lithium iron phosphate material at room temperature and pressure according to claim 1, characterized in that: In step (4), the lithium source is one or any combination of lithium phosphate, lithium carbonate, and lithium hydroxide; the iron source is one or any combination of iron powder and ferrous sulfate; and the phosphorus source is one or any combination of phosphoric acid, lithium phosphate, and ammonium hydrogen phosphate.

3. The method for synthesizing LFP precursor using waste lithium iron phosphate material at room temperature and pressure according to claim 1, characterized in that: The molar ratio of lithium, iron and phosphorus in step (4) is adjusted in the range of (0.6-1.00):1:(0.6-1.00).

4. The method for synthesizing LFP precursor using waste lithium iron phosphate material at room temperature and pressure according to claim 1, characterized in that: The lithium source used to prepare the base solution E in step (5) can be one or any combination of lithium carbonate, lithium dihydrogen phosphate, lithium sulfate, and lithium hydroxide; the phosphorus source used to prepare the base solution E in step (5) can be any two or more of phosphoric acid, lithium phosphate, and ammonium hydrogen phosphate.

5. The method for synthesizing LFP precursor using waste lithium iron phosphate material at room temperature and pressure according to claim 1, characterized in that: In step (6), one or any combination of alkaline substances selected from the group consisting of liquid alkali, lithium carbonate, lithium hydroxide, sodium carbonate, ammonium carbonate, ammonium bicarbonate and aqueous ammonia is used to control the pH of the reaction system to be in the range of 5.0-7.

0.

6. The method for synthesizing LFP precursor using waste lithium iron phosphate material at room temperature and pressure according to claim 1, characterized in that: The lithium iron phosphorus mixed solution D in step (6) is added to the bottom solution E for 4-8 hours.

7. The method for synthesizing LFP precursor using waste lithium iron phosphate materials at room temperature and pressure according to claim 1, characterized in that: The aging time after the reaction in step (6) is 4-8 hours.

8. The method for synthesizing LFP precursor using waste lithium iron phosphate materials at room temperature and pressure according to claim 1, characterized in that: The mass of the carbon source added in step (8) is 5-7% of the mass of the precursor.

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