Preparation method of lithium iron phosphate positive electrode material

Abstract

The invention discloses a preparation method of a lithium iron phosphate positive electrode material, and belongs to the technical field of lithium ion batteries. The method comprises the following steps: discharging, disassembling and sorting to obtain a positive plate, a negative plate, a diaphragm, a shell and a current collector, carrying out freeze drying or negative-pressure heating, stirring and drying treatment on the positive plate to obtain waste lithium iron phosphate powder, carrying out ICP analysis on the waste lithium iron phosphate powder, adjusting the molar ratio of lithium to iron to phosphorus to be (1-1.1): 1: 1, adding a carbon source and alcohol, carrying out ball milling, drying, and calcining in a protective atmosphere to obtain the lithium iron phosphate. The lithium iron phosphate positive electrode material is obtained. Compared with a traditional method, the lithium iron phosphate positive electrode material obtained by the method is low in aluminum content and impurity content, and the battery has good electrochemical performance when the lithium iron phosphate positive electrode material is assembled into a button cell to be subjected to a charge-discharge test.

Description

Technical Field

[0001] The invention relates to a method for preparing a lithium iron phosphate positive electrode material, and belongs to the technical field of lithium ion batteries. Background Art

[0002] The popularization of new energy vehicles is a strategic choice to promote green development and ensure energy security, and it is of great significance for my country to achieve the goals of carbon peak and carbon neutrality. Lithium-ion batteries are the core driving force of new energy vehicles. The waste generated during the battery manufacturing process and its clean treatment and efficient utilization after retirement are related to whether the electric vehicle industry can achieve green and sustainable development. High-value recycling of them also has certain economic value. At present, the most widely used method of battery recycling in China is hydrometallurgy.

[0003] The wet recovery process is a process in which the pretreated positive electrode material is leached into the solution in the form of ions, and then concentrated and purified to generate metal salts or other products. After the waste batteries are sorted, classified, and shelled, they need to be reduced and leached. Reduction leaching often uses the acid leaching process, which can dissolve almost all transition metal oxides into the solution, and usually uses strong inorganic acids as leaching agents. Since the use of strong inorganic acids as leaching agents will cause Cl2, SO x and NO x The emission of toxic gases and the generation of waste acid liquid, the cost of absorption equipment and purification equipment is high, so organic acids such as citric acid or malic acid can also be used instead. Compared with the pyrometallurgical recovery process, the wet recovery process has the advantages of higher recovery rate, higher product purity, and lower operating temperature, and has become the most commonly used method for recycling waste batteries. However, wet recovery also has disadvantages such as long process flow, complicated operating procedures, and high pollution control costs. Since excessive corrosive solutions such as acid are generally used in the wet recovery process to ensure leaching efficiency, a lot of waste will be caused, and the waste liquid is difficult to handle, which may cause secondary pollution. The lithium iron phosphate material itself has a stable structure and can be directly recycled and reused. The main reason for the failure of lithium iron phosphate is the lack of lithium, which can be repaired and regenerated by supplementing the lithium source and then used directly.

[0004] A Chinese patent document with publication number CN106276842A discloses a "method for recovering lithium iron phosphate from waste lithium-ion batteries". This method adds an organic acid to waste lithium iron phosphate powder, treats it at a certain temperature, and adds an organic solvent to the resulting acidic solution after filtering to obtain a lithium iron phosphate precursor precipitate. This method requires the use of organic solvents and organic acids, which will cause certain pollution to the environment and has high production costs.

[0005] A Chinese patent document with publication number CN106276842A discloses "a method for recycling and treating waste lithium iron phosphate batteries". In this method, calcined powder is slurried, mixed acid is added for leaching, and a leachate and a leaching residue are obtained by filtering; iron powder is added to the leachate for precipitation reaction, and a copper removal solution is obtained by filtering; the copper removal solution is adjusted to have a pH value for precipitation reaction, and an aluminum removal solution is obtained by filtering; a complexing agent and a reducing agent are added to the aluminum removal solution, and the pH value is adjusted for precipitation reaction to obtain a lithium iron phosphate precursor slurry; after the lithium iron phosphate precursor slurry is dried, a carbon source is added for calcination to obtain a lithium iron phosphate positive electrode material. This method uses acid leaching, and ammonia water is used as a pH adjuster. A large amount of acid and alkali is used, which will result in high raw material costs and environmental pollution.

[0006] A Chinese patent document with publication number CN102751548A discloses "a method for recovering and preparing lithium iron phosphate from waste lithium iron phosphate batteries". The method comprises disassembling the recycled waste lithium iron phosphate batteries, removing the battery shells, and then soaking them. The pole pieces are separated from the positive and negative electrode powders by alternating ultrasound and mechanical stirring, and the pole pieces and diaphragms are taken out to obtain a mixed solution. The mixed solution is preliminarily ball-milled, and the slurry after the preliminary ball milling and mixing is dried. The dried material is calcined in an oxidizing atmosphere to obtain a recycled material, and an iron source and a phosphorus source are added, and a new lithium iron phosphate raw material is optionally added, and then a carbon source is added for high-energy ball milling. After drying, a recycled waste pre-sintered material is obtained. The recycled waste pre-sintered material is subjected to microwave sintering or solid-phase sintering treatment under a protective atmosphere to obtain a lithium iron phosphate product. The method uses an alternating ultrasound and mechanical stirring method to separate the pole pieces from the positive and negative electrode powders. The aluminum impurity content obtained by this method will be high, which affects the electrochemical properties of the subsequent preparation of lithium iron phosphate positive electrode materials. Summary of the invention

[0007] The object of the present invention is to provide a method for preparing a lithium iron phosphate positive electrode material. The lithium iron phosphate positive electrode material prepared by the method has low impurity content and high specific capacity.

[0008] In order to solve the above technical problems, the technical solution of the present invention is: A method for preparing a lithium iron phosphate positive electrode material comprises the following steps: (1) After discharge, disassembly and sorting, the waste lithium iron phosphate batteries are obtained into positive electrode sheets, negative electrode sheets, separators, current collectors and metal casings; (2) freeze-drying or negative pressure heating and stirring drying the positive electrode sheet to obtain waste lithium iron phosphate sheet and aluminum foil; (3) Grinding and sieving the waste lithium iron phosphate sheets to obtain waste lithium iron phosphate powder; (4) performing ICP analysis on the waste lithium iron phosphate powder, and adding at least one of a lithium source, a phosphorus source or an iron source to the waste lithium iron phosphate powder according to the ICP analysis result to obtain a mixed material, controlling the molar ratio of lithium, iron and phosphorus in the mixed material to be 1-1.1:1:1, and then adding a carbon source to the mixed material to obtain a mixed material, and wet-grinding and drying the mixed material to obtain a pretreated lithium iron phosphate regenerated material, wherein the amount of the carbon source added is 5%-30% of the mass of the waste lithium iron phosphate powder; (5) Under a protective atmosphere, the pretreated lithium iron phosphate recycled material is calcined at a high temperature to obtain a lithium iron phosphate positive electrode material.

[0009] Preferably, in step (2), the specific process of freeze-drying the positive electrode sheet is to place the positive electrode sheet in a freezing environment, set the temperature to -60 to -20°C, take it out after 0.5 to 10 hours, and then dry and sort the positive electrode sheet to obtain waste lithium iron phosphate sheets and aluminum foil.

[0010] Preferably, in the step (2), the specific process of subjecting the positive electrode sheet to negative pressure heating, stirring and drying treatment is as follows: placing the positive electrode sheet in deionized water, subjecting it to negative pressure heating and stirring, setting the temperature to 40-70°C and the pressure to -0.08MPa--0.03MPa, taking it out after 0.5-5h, and then drying and sorting the positive electrode sheet to obtain waste lithium iron phosphate sheet and aluminum foil.

[0011] Preferably, in step (3), the particle size of the waste lithium iron phosphate powder is 40-200 mesh.

[0012] Preferably, in step (4), the carbon source is any one of glucose or sucrose or citric acid or starch; the lithium source is any one of lithium carbonate or lithium hydroxide or lithium acetate or lithium oxalate or lithium nitrate; the phosphorus source is any one of diammonium phosphate or phosphoric acid or diammonium hydrogen phosphate or ferrous phosphate; and the iron source is any one of ferrous oxalate dihydrate or ferrous phosphate or ferrous acetate.

[0013] Preferably, in step (5), the specific process of high temperature roasting is to first increase the roasting temperature to 250-400°C and keep it at that temperature for 1.5-3 hours; then increase the temperature to 650-800°C and keep it at that temperature for 5-8 hours.

[0014] The beneficial effects of the present invention are as follows: The traditional ultrasonic hydrothermal process uses external energy to reduce the adhesion between lithium iron phosphate and aluminum foil and induces separation, but it will induce side reactions and promote the formation of impurities. The technical solution of the present invention reduces the adhesion between waste lithium iron phosphate sheets and aluminum foil by freezing; or heats and stirs under negative pressure conditions. Under negative pressure conditions, the adhesion between waste lithium iron phosphate sheets and aluminum foil can be reduced, and the two are separated under the action of heated and stirred water; compared with the traditional ultrasonic hydrothermal process, the waste lithium iron phosphate sheets and aluminum foil are separated by freezing separation or negative pressure heating and stirring, and the waste lithium iron phosphate powder obtained has a low content of aluminum impurities, and the prepared lithium iron phosphate positive electrode material has good electrochemical properties.

[0015] The technical solution of the present invention does not use chemicals such as acids, alkalis and organic solvents, does not generate pollution in the entire process, and is harmless to the environment. It is a green comprehensive utilization process. The entire process is simple to operate and can retain the aluminum foil to the greatest extent. The waste battery recovery rate is high, the production cost is low, and it has good environmental protection and economic benefits. DETAILED DESCRIPTION Example 1

[0016] Discharge the waste lithium iron phosphate battery until the discharge voltage is zero, disassemble and sort to obtain the positive electrode sheet, negative electrode sheet, diaphragm, current collector and metal shell; The positive electrode sheet of the waste lithium iron phosphate battery is placed in a freezing environment, the temperature is set to -60°C, and the positive electrode sheet is taken out after freezing for 0.5 hours, and then the positive electrode sheet is placed in a freeze dryer for drying to obtain a mixture of aluminum foil and waste lithium iron phosphate sheets, and the aluminum foil and the waste lithium iron phosphate sheets are separated by manual sorting, and then the waste lithium iron phosphate sheets are ground and passed through a 60-mesh molecular sieve to obtain waste lithium iron phosphate powder; The waste lithium iron phosphate powder is subjected to ICP analysis, and the molar ratio of lithium, iron and phosphorus in the waste lithium iron phosphate powder is obtained by elemental analysis, and then lithium carbonate and ferrous oxalate dihydrate are added to obtain a mixed material; the molar ratio of lithium, iron and phosphorus in the mixed material is adjusted to 1.05:1:1, and then glucose with a mass of 10% of the mass of the waste lithium iron phosphate powder is added to the mixed material; a mixed material is obtained, the mixed material is put into a ball mill, and then alcohol and zirconium oxide balls are added to the ball mill, the amount of alcohol added is 2 times the mass of the mixed material, and the amount of zirconium oxide balls added is 20 times the mass of the mixed material, and the mixed material is wet-milled at a wet-milling speed of 500rpm / min. After wet-milling for 4 hours, the mixed material is taken out and dried to obtain a pretreated lithium iron phosphate recycled material; The pretreated lithium iron phosphate regenerated material was placed in argon gas and first heated to 350°C and calcined for 2 hours, and then heated to 650°C and calcined for 6 hours to obtain a lithium iron phosphate positive electrode material, named 1# positive electrode material. Example 2

[0017] Discharge the waste lithium iron phosphate battery until the discharge voltage is zero, disassemble and sort to obtain the positive electrode sheet, negative electrode sheet, diaphragm, current collector and metal shell; The positive electrode sheet of the waste lithium iron phosphate battery is placed in deionized water, and negative pressure heating and stirring are performed. The temperature is set to 40°C and the pressure is -0.08MPa. After 2.5 hours, the positive electrode sheet is taken out, and then the positive electrode sheet is dried and sorted to obtain a mixture of waste lithium iron phosphate sheets and aluminum foil. The aluminum foil and the waste lithium iron phosphate sheet are separated by manual sorting, and then the waste lithium iron phosphate sheet is ground and passed through a 40-mesh molecular sieve to obtain waste lithium iron phosphate powder; The waste lithium iron phosphate powder is subjected to ICP analysis, and the molar ratio of lithium, iron and phosphorus in the waste lithium iron phosphate powder is obtained by elemental analysis, and then lithium carbonate and ammonium dihydrogen phosphate are added to obtain a mixed material; the molar ratio of lithium, iron and phosphorus in the mixed material is adjusted to 1:1:1, and then glucose with a mass of 5% of the mass of the waste lithium iron phosphate powder is added to the mixed material; a mixed material is obtained, the mixed material is put into a ball mill, and then alcohol and zirconium oxide balls are added to the ball mill, the amount of the alcohol added is 5 times the mass of the mixed material, and the amount of the zirconium oxide balls added is 60 times the mass of the mixed material, and the mixed material is wet-milled at a speed of 300rpm / min. After wet-milling for 8 hours, the mixed material is taken out and dried to obtain a pretreated lithium iron phosphate recycled material; The pretreated lithium iron phosphate regenerated material was placed in argon gas and first heated to 250°C and calcined for 3 hours, and then heated to 650°C and calcined for 8 hours to obtain a lithium iron phosphate positive electrode material, named 2# positive electrode material. Example 3

[0018] Discharge the waste lithium iron phosphate battery until the discharge voltage is zero, disassemble and sort to obtain the positive electrode sheet, negative electrode sheet, diaphragm, current collector and metal shell; The positive electrode sheet of the waste lithium iron phosphate battery is placed in deionized water, and negative pressure heating and stirring are performed. The temperature is set to 70°C and the pressure is -0.03MPa. After 5 hours, the positive electrode sheet is taken out, and then the positive electrode sheet is dried and sorted to obtain a mixture of waste lithium iron phosphate sheets and aluminum foil. The aluminum foil and the waste lithium iron phosphate sheet are separated by manual sorting, and then the waste lithium iron phosphate sheet is ground and passed through a 200-mesh molecular sieve to obtain waste lithium iron phosphate powder; The waste lithium iron phosphate powder is subjected to ICP analysis, and the molar ratio of lithium, iron and phosphorus in the waste lithium iron phosphate powder is obtained by elemental analysis, and then lithium hydroxide and ferric nitrate are added to obtain a mixed material; the molar ratio of lithium, iron and phosphorus in the mixed material is adjusted to 1.1:1:1, and sucrose with a mass of 20% of the mass of the waste lithium iron phosphate powder is added to the mixed material; a mixed material is obtained, the mixed material is put into a ball mill, and alcohol and zirconium oxide balls are added to the ball mill, the amount of alcohol added is 2 times the mass of the mixed material, and the amount of zirconium oxide balls added is 10 times the mass of the mixed material, and the mixed material is wet-milled at a speed of 700rpm / min. After wet-milling for 1 hour, the mixed material is taken out and dried to obtain a pretreated lithium iron phosphate recycled material; The pretreated lithium iron phosphate regenerated material was placed in argon gas and first heated to 400°C and calcined for 1.5 hours, and then heated to 800°C and calcined for 5 hours to obtain a lithium iron phosphate positive electrode material, named 3# positive electrode material. Example 4

[0019] Discharge the waste lithium iron phosphate battery until the discharge voltage is zero, disassemble and sort to obtain the positive electrode sheet, negative electrode sheet, diaphragm, current collector and metal shell; The positive electrode sheet of the waste lithium iron phosphate battery is placed in deionized water, and negative pressure heating and stirring are performed. The temperature is set to 60°C and the pressure is -0.07MPa. After 0.5h, the positive electrode sheet is taken out, and then the positive electrode sheet is dried and sorted to obtain a mixture of waste lithium iron phosphate sheets and aluminum foil. The aluminum foil and the waste lithium iron phosphate sheet are separated by manual sorting, and then the waste lithium iron phosphate sheet is ground and passed through an 80-mesh molecular sieve to obtain waste lithium iron phosphate powder; The waste lithium iron phosphate powder is subjected to ICP analysis, and the molar ratio of lithium, iron and phosphorus in the waste lithium iron phosphate powder is obtained by elemental analysis, and then lithium carbonate and ferrous oxalate dihydrate are added to obtain a mixed material; the molar ratio of lithium, iron and phosphorus in the mixed material is adjusted to 1.05:1:1, and sucrose with a mass of 30% of the mass of the waste lithium iron phosphate powder is added to the mixed material; a mixed material is obtained, the mixed material is put into a ball mill, and alcohol and zirconium oxide balls are added to the ball mill, the amount of alcohol added is 2 times the mass of the mixed material, and the amount of zirconium oxide balls added is 20 times the mass of the mixed material, and the mixed material is wet-milled at a speed of 600rpm / min. After wet-milling for 3 hours, the mixed material is taken out and dried to obtain a pretreated lithium iron phosphate recycled material; The pretreated lithium iron phosphate regenerated material was placed in argon gas and first heated to 350°C and calcined for 2.5 hours, and then heated to 650°C and calcined for 8 hours to obtain a lithium iron phosphate positive electrode material, named 4# positive electrode material. Example 5

[0020] Discharge the waste lithium iron phosphate battery until the discharge voltage is zero, disassemble and sort to obtain the positive electrode sheet, negative electrode sheet, diaphragm, current collector and metal shell; The positive electrode sheet of the waste lithium iron phosphate battery is placed in a freezing environment, the temperature is set to -20°C, and the positive electrode sheet is taken out after freezing for 10 hours, and then the positive electrode sheet is placed in a freeze dryer for drying to obtain a mixture of aluminum foil and waste lithium iron phosphate sheets, and the aluminum foil and the waste lithium iron phosphate sheets are separated by manual sorting, and then the waste lithium iron phosphate sheets are ground and passed through a 100-mesh molecular sieve to obtain waste lithium iron phosphate powder; The waste lithium iron phosphate powder is subjected to ICP analysis, and the molar ratio of lithium, iron and phosphorus in the waste lithium iron phosphate powder is obtained by elemental analysis, and then lithium hydroxide and ferrous oxalate dihydrate are added to obtain a mixed material; the molar ratio of lithium, iron and phosphorus in the mixed material is adjusted to 1.02:1:1, and then glucose with a mass of 15% of the mass of the waste lithium iron phosphate powder is added to the mixed material; a mixed material is obtained, the mixed material is put into a ball mill, and then alcohol and zirconium oxide balls are added to the ball mill, the amount of alcohol added is 2 times the mass of the mixed material, and the amount of zirconium oxide balls added is 30 times the mass of the mixed material, and the mixed material is wet-milled at a speed of 400rpm / min. After wet-milling for 5 hours, the mixed material is taken out and dried to obtain a pretreated lithium iron phosphate recycled material; The pretreated lithium iron phosphate regenerated material was placed in argon gas and first heated to 400°C and calcined for 1.5 hours, and then heated to 700°C and calcined for 6 hours to obtain a lithium iron phosphate positive electrode material, named 5# positive electrode material. Example 6

[0021] Discharge the waste lithium iron phosphate battery until the discharge voltage is zero, disassemble and sort to obtain the positive electrode sheet, negative electrode sheet, diaphragm, current collector and metal shell; The positive electrode sheet of the waste lithium iron phosphate battery is placed in a freezing environment, the temperature is set to -40°C, and the positive electrode sheet is taken out after freezing for 5 hours, and then the positive electrode sheet is placed in a freeze dryer for drying to obtain a mixture of aluminum foil and waste lithium iron phosphate sheets, and the aluminum foil and the waste lithium iron phosphate sheets are separated by manual sorting, and then the waste lithium iron phosphate sheets are ground and passed through a 100-mesh molecular sieve to obtain waste lithium iron phosphate powder; The waste lithium iron phosphate powder is subjected to ICP analysis, and the molar ratio of lithium, iron and phosphorus in the waste lithium iron phosphate powder is obtained by elemental analysis, and then lithium hydroxide and ferrous oxalate dihydrate are added to obtain a mixed material; the molar ratio of lithium, iron and phosphorus in the mixed material is adjusted to 1.05:1:1, and then glucose with a mass of 15% of the mass of the waste lithium iron phosphate powder is added to the mixed material; a mixed material is obtained, the mixed material is put into a ball mill, and then alcohol and zirconium oxide balls are added to the ball mill, the amount of alcohol added is 2 times the mass of the mixed material, and the amount of zirconium oxide balls added is 30 times the mass of the mixed material, and the mixed material is wet-milled at a speed of 400rpm / min. After wet-milling for 5 hours, the mixed material is taken out and dried to obtain a pretreated lithium iron phosphate recycled material; The pretreated lithium iron phosphate regenerated material was placed in argon gas and first heated to 400°C and calcined for 1.5 hours, and then heated to 700°C and calcined for 6 hours to obtain a lithium iron phosphate positive electrode material, named 6# positive electrode material. (Using traditional ultrasonic hydrothermal treatment process to separate aluminum foil and waste lithium iron phosphate sheets)

[0022] Discharge the waste lithium iron phosphate battery until the discharge voltage is zero, disassemble and sort to obtain the positive electrode sheet, negative electrode sheet, diaphragm, current collector and metal shell; The positive electrode sheet of the waste lithium iron phosphate battery is subjected to conventional ultrasonic hydrothermal treatment to obtain a mixture of aluminum foil and waste lithium iron phosphate sheets, the aluminum foil and the waste lithium iron phosphate sheets are separated by manual sorting, and the waste lithium iron phosphate sheets are ground and passed through a 60-mesh molecular sieve to obtain waste lithium iron phosphate powder; The waste lithium iron phosphate powder is subjected to ICP analysis, and the molar ratio of lithium, iron and phosphorus in the waste lithium iron phosphate powder is obtained by elemental analysis, and then lithium carbonate and ferrous oxalate dihydrate are added to obtain a mixed material; the molar ratio of lithium, iron and phosphorus in the mixed material is adjusted to 1.05:1:1, and then glucose with a mass of 10% of the mass of the waste lithium iron phosphate powder is added to the mixed material; a mixed material is obtained, the mixed material is put into a ball mill, and then alcohol and zirconium oxide balls are added to the ball mill, the amount of alcohol added is 2 times the mass of the mixed material, and the amount of zirconium oxide balls added is 20 times the mass of the mixed material, and the mixed material is wet-milled at a wet-milling speed of 500rpm / min. After wet-milling for 4 hours, the mixed material is taken out and dried to obtain a pretreated lithium iron phosphate recycled material; The pretreated lithium iron phosphate regenerated material was placed in argon gas, first heated to 350°C and calcined for 2 hours, and then heated to 650°C and calcined for 6 hours to obtain a lithium iron phosphate positive electrode material, named 7# positive electrode material. Test Example 1

[0023] The aluminum impurity content in the waste lithium iron phosphate powder used in Examples 1-5 and Comparative Example 1 was measured by inductively coupled plasma emission spectroscopy (ICP), by spraying the decomposed metal ion solution into a plasma flame, and using the energy emitted by the excited particles to compare with the standard solution, thereby obtaining the type and content of the element. In the present invention, the waste lithium iron phosphate powder was leached in aqua regia and microwaved for 30 minutes at 60°C, and then the metal element content was tested; the test data is shown in Table 1. Test Example 2

[0024] The lithium iron phosphate positive electrode materials prepared in Examples 1-5 and Comparative Example 1 are made into button batteries, specifically including the following steps: According to the mass ratio of 8:1:1, the lithium iron phosphate positive electrode material, binder powder (PVDF), and conductive agent powder (SuperP) are weighed respectively, and evenly ground and mixed. A certain amount of binder powder solvent (NMP) is added, and the mixed powder is ground into a uniform slurry with a solid content of 58%, and the slurry is coated on the non-smooth surface of the aluminum foil so that the positive electrode coating and the aluminum foil are tightly bonded. The coated aluminum foil is placed in a vacuum drying oven at 60°C and dried for 6 hours to remove NMP. Check whether the dried pole piece has powdering and cracking, and place the pole piece with good appearance on the punching machine for slicing to make the positive pole piece dry for standby use. The lithium sheet is used as the counter electrode of the regenerated positive electrode, 1 mol / L LiPF6 / EC:DMC (1:1) is used as the electrolyte in the button battery, and the Celgard2325 composite film is used as the battery separator. The assembled button battery parts are placed on the battery sealing machine mold for compression packaging, then removed from the glove box and placed horizontally for 6-12 hours to allow the electrolyte to fully penetrate the inside of the battery. The electrochemical performance tests and analyses are then performed on the Xinwei constant temperature test system. The test data are shown in Table 1.

[0025]

[0026] It can be seen from Table 1 that the aluminum content in the waste lithium iron phosphate powder obtained by freeze drying separation or negative pressure heating and stirring is significantly less than the aluminum content in the waste lithium iron phosphate powder obtained by ultrasonic hydrothermal separation. Finally, the button battery prepared from the lithium iron phosphate positive electrode material obtained from the waste lithium iron phosphate powder has a higher discharge specific capacity. This is because the mechanism of ultrasonic hydrothermal separation is mainly the promotion of water convection by ultrasonic waves, which forms liquid phase cavitation in a short time and produces micro bubbles with high surface activity and a large amount of energy. These have caused the chemical bond between the binder and the aluminum foil to break and weaken the binding force, but will induce side reactions and cause the formation of impurities. Freeze drying reduces the binding force between the binder and the aluminum foil under low temperature conditions, thereby separating without inducing side reactions, so there are fewer aluminum impurities; under negative pressure conditions, the bonding force between the waste lithium iron phosphate sheet and the aluminum foil can be reduced, and the two are separated under the action of heated and stirred water, so there are fewer aluminum impurities, and the lithium iron phosphate positive electrode material prepared from the waste lithium iron phosphate powder with higher purity has better electrochemical properties.

[0027] At the same time, in this invention, no chemicals such as acid, alkali and organic solvents are used, no pollution is generated in the entire process, and there is no harm to the environment. It is a green comprehensive utilization process. The entire process is simple to operate and can retain the aluminum foil to the greatest extent. The recycling rate of waste batteries is high, the production cost is low, and it has good environmental and economic benefits.

Claims

1. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The steps include: (1) After discharge, disassembly and sorting, waste lithium iron phosphate batteries are obtained to obtain positive electrode sheets, negative electrode sheets, separators, current collectors and metal casings; (2) freeze-drying or negative pressure heating and stirring drying the positive electrode sheet to obtain waste lithium iron phosphate sheet and aluminum foil; (3) Grinding and sieving the waste lithium iron phosphate sheets to obtain waste lithium iron phosphate powder; (4) performing ICP analysis on the waste lithium iron phosphate powder, and adding at least one of a lithium source, a phosphorus source or an iron source to the waste lithium iron phosphate powder according to the ICP analysis result to obtain a mixed material, controlling the molar ratio of lithium, iron and phosphorus in the mixed material to be 1-1.1:1:1, and then adding a carbon source to the mixed material to obtain a mixed material, and wet-grinding and drying the mixed material to obtain a pretreated lithium iron phosphate regenerated material, wherein the amount of the carbon source added is 5%-30% of the mass of the waste lithium iron phosphate powder; (5) Under a protective atmosphere, the pretreated lithium iron phosphate recycled material is calcined at a high temperature to obtain a lithium iron phosphate positive electrode material.

2. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: In the step (2), the specific process of freeze-drying the positive electrode sheet is to place the positive electrode sheet in a freezing environment, set the temperature to -60 to -20°C, take it out after 0.5 to 10 hours, and then dry and sort the positive electrode sheet to obtain waste lithium iron phosphate sheets and aluminum foil.

3. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: In the step (2), the specific process of subjecting the positive electrode sheet to negative pressure heating, stirring and drying treatment is as follows: placing the positive electrode sheet in deionized water, subjecting it to negative pressure heating and stirring, setting the temperature to 40-70°C and the pressure to -0.08MPa--0.03MPa, taking it out after 0.5-5h, and then drying and sorting the positive electrode sheet to obtain waste lithium iron phosphate sheet and aluminum foil.

4. The method for preparing a lithium iron phosphate positive electrode material according to claim 1, characterized in that: In the step (3), the particle size of the waste lithium iron phosphate powder is 40-200 mesh.

5. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: In the step (4), the carbon source is any one of glucose, sucrose, citric acid or starch; the lithium source is any one of lithium carbonate, lithium hydroxide, lithium acetate, lithium oxalate or lithium nitrate; the phosphorus source is any one of diammonium phosphate, phosphoric acid, diammonium hydrogen phosphate or ferrous phosphate; and the iron source is any one of ferrous oxalate dihydrate, ferrous phosphate or ferrous acetate.

6. The method for preparing a lithium iron phosphate positive electrode material according to claim 1, characterized in that: In the step (5), the specific process of high temperature roasting is to first increase the roasting temperature to 250-400°C and keep it at that temperature for 1.5-3 hours; then increase the temperature to 650-800°C and keep it at that temperature for 5-8 hours.

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