Method for preparing lithium iron phosphate by recycling waste lithium iron phosphate material

Abstract

The invention provides a method for preparing lithium iron phosphate by recovering a waste lithium iron phosphate material, which comprises the following steps: crushing a waste lithium iron phosphate battery positive plate, vibrating and sieving to obtain a lithium iron phosphate raw material, dissolving in an acid solution, filtering, and collecting a first filtrate; adding a ferrous sulfate solution into the first filtrate to obtain a mixed solution; adding hydrogen peroxide and an ammonia water solution into the mixed solution to adjust the pH value, filtering and washing after reaction to respectively obtain a second filtrate and a filter cake, and washing and drying the filter cake to obtain iron hydroxyphosphate; adding a barium hydroxide solution into the second filtrate, and filtering after complete reaction to obtain a third filtrate; adding a phosphoric acid solution into the third filtrate to obtain fourth filtrate; mixing the hydroxyl iron phosphate, the fourth filtrate and a carbon source, sanding the slurry, and drying to obtain powder; and sintering the powder in an inert atmosphere, naturally cooling and crushing to obtain the carbon-coated lithium iron phosphate material.

Description

Technical Field

[0001] The present invention relates to the technical field of methods for preparing positive electrode materials for lithium-ion batteries, and in particular to a method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials. Background Art

[0002] Lithium iron phosphate cathode material is the fastest growing lithium battery cathode material in China. Its raw materials are widely available and inexpensive. It is widely used in the domestic battery industry in the fields of automobiles, power tools, energy storage equipment, emergency power supply equipment and mobile power supplies. Among them, new energy electric vehicles are the main application field. Compared with other batteries, lithium iron phosphate batteries have the advantages of safety, environmental protection, low cost, long cycle life and good high temperature performance. Since the life of lithium iron phosphate batteries is generally 2-10 years, a large amount of waste lithium iron phosphate materials will be generated over time. In order to recycle and reuse waste lithium iron phosphate materials, it is currently mainly divided into dry recycling and wet recycling. The dry recycling process is simple and widely used, but it consumes a lot of energy, wastes resources, and produces polluting gases or substances during the production process. The wet recycling process is stable, but wet recycling requires a large amount of liquid alkali and oxidant, and the waste liquid needs to be further processed in the later stage. The recycling cost is also high, and the iron, phosphorus, lithium and other materials in the waste lithium iron phosphate materials are not fully recovered.

[0003] Therefore, the existing recycling methods have the problems of high recycling cost and low recycling efficiency of lithium iron phosphate materials, and only specifically recover precious metal ions, thereby wasting iron and phosphorus materials in the lithium iron phosphate material and failing to fully recycle the lithium iron phosphate material. Summary of the invention

[0004] In view of the above, the present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention proposes a method for recycling waste lithium iron phosphate materials to prepare lithium iron phosphate. The method provided by the present invention can fully recycle phosphorus, iron and lithium elements in waste lithium iron phosphate materials, and use the recovered phosphorus, iron and lithium as raw materials for preparing lithium iron phosphate, and the prepared nano-scale lithium iron phosphate material has a higher discharge specific capacity. In addition, the method has high production efficiency, low production cost, and is environmentally friendly, suitable for application in large-scale industrial production, and has high economic and social benefits.

[0005] To this end, an embodiment of the present invention provides a method for recycling waste lithium iron phosphate materials to prepare lithium iron phosphate, the method comprising: crushing waste lithium iron phosphate battery positive electrode sheets, shaking and screening to obtain lithium iron phosphate raw materials, dissolving the lithium iron phosphate raw materials in an acidic solution, filtering and collecting a first filtrate; adding ferrous sulfate solution to the first filtrate to obtain a mixed solution; adding hydrogen peroxide and ammonia solution to the mixed solution to adjust the pH value, filtering and washing after reaction, respectively obtaining a second filtrate and a filter cake, washing and drying the filter cake to obtain hydroxy iron phosphate; adding barium hydroxide solution to the second filtrate, filtering after the reaction is complete, obtaining a third filtrate; adding phosphoric acid solution to the third filtrate to obtain a fourth filtrate; mixing the hydroxy iron phosphate, the fourth filtrate and a carbon source, sand-grinding the slurry, and obtaining a powder after drying; sintering the powder in an inert atmosphere, and crushing it after natural cooling to obtain a carbon-coated lithium iron phosphate material.

[0006] Preferably, the acidic solution may be a sulfuric acid solution or a phosphoric acid solution, and the concentration of the acidic solution is greater than 30 wt %.

[0007] Preferably, the mass ratio of the added acidic solution to the lithium iron phosphate raw material is [3-5]:1.

[0008] Preferably, the amount of ferrous sulfate solution added is based on the iron-phosphorus molar ratio in the mixed solution satisfying: Fe / P=1.45-1.5.

[0009] Preferably, the concentration of the hydrogen peroxide solution is 20-30 wt %, the concentration of the ammonia solution is 20-30 wt %, and the pH value is adjusted to 2.0-4.5 by adding the hydrogen peroxide solution and the ammonia solution.

[0010] Preferably, a barium hydroxide solution is added to remove sulfate ions in the second filtrate.

[0011] Preferably, the amount of phosphoric acid solution added is based on the lithium-phosphorus molar ratio in the fourth filtrate satisfying: Li / P=2.8-3.2.

[0012] Preferably, after the hydroxyferric phosphate and the fourth filtrate are mixed, the molar ratio of iron element to phosphorus element satisfies n(Fe):n(Li)=1:[1-1.04]; the amount of the carbon source added is based on the carbon content in the lithium iron phosphate material being between 0.5-3wt%, and the carbon source is a mixture of glucose and polyethylene glycol; the slurry particle size is 0.40-0.95μm.

[0013] Preferably, the amount of the carbon source added is based on the carbon content in the lithium iron phosphate material being between 0.8-2 wt %, the slurry particle size is 0.50-0.75 μm, and the powder particle size is 20-40 μm.

[0014] Preferably, the gas used in the inert atmosphere is selected from at least one of nitrogen, argon and helium; the sintering temperature may be 750-900° C., and the sintering time may be 5-12 hours; and the particle size of the carbon-coated lithium iron phosphate material is 0.80-3 μm.

[0015] The method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials provided by the embodiment of the present invention, when the waste lithium iron phosphate materials are processed, the iron element and the phosphorus element are recovered in the form of hydroxy iron phosphate by adjusting the amount of iron source added and the pH value, and the precipitation amount of the lithium element when the phosphorus element and the iron element are precipitated is minimized, and the lithium element is separated from the phosphorus element and the iron element; the method provided by the present invention recovers the phosphorus element and the iron element in the form of hydroxy iron phosphate, and the lithium element is recovered in the form of lithium hydroxide, thereby improving the recovery rate of the lithium element, the phosphorus element and the iron element. When the lithium iron phosphate material is subsequently prepared, phosphoric acid is added to the filtrate, which can not only adjust the Li / P ratio to a suitable range, but also prevent the lithium element from precipitating, so that the lithium element can be recovered in the filtrate in the form of lithium hydroxide. And the method has high production efficiency, low production cost, and is environmentally friendly, suitable for application in large-scale industrial production, and has high economic and social benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 A flow chart of a method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials provided in an embodiment of the present invention;

[0017] Figure 2 A process flow chart for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials provided in an embodiment of the present invention;

[0018] Figure 3 This is a SEM spectrum of the lithium iron phosphate material prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0019] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

[0020] The disclosure below provides many different embodiments or examples to implement different structures of the present invention. In order to simplify the disclosure of the present invention, the parts and settings of specific examples are described below. Of course, they are only examples, and the purpose is not to limit the present invention. In addition, the present invention can repeat reference numbers and / or letters in different examples. This repetition is for the purpose of simplicity and clarity, and does not itself indicate the relationship between the various embodiments and / or settings discussed. In addition, the present invention provides examples of various specific processes and materials, but those of ordinary skill in the art can appreciate the reusability of other processes and / or the use of other materials.

[0021] The embodiment of the present invention provides a method for recycling waste lithium iron phosphate materials to prepare lithium iron phosphate. The method separately recovers phosphorus, iron and lithium elements in waste lithium iron phosphate batteries, recovers all phosphorus and iron elements in the waste lithium iron phosphate batteries in the form of hydroxy iron phosphate; all lithium elements are present in the form of lithium hydroxide in the recovered filtrate, and finally the recovered hydroxy iron phosphate and lithium-containing filtrate are used as raw materials to prepare nano-scale lithium iron phosphate. Figure 1 and Figure 2 As shown, the method includes:

[0022] Step S1: crushing the waste lithium iron phosphate battery positive electrode sheet, shaking and sieving to obtain a lithium iron phosphate raw material, dissolving the lithium iron phosphate raw material in an acidic solution, filtering and collecting a first filtrate;

[0023] The acidic solution may be a sulfuric acid solution or a phosphoric acid solution, and the concentration of the acidic solution is greater than 30wt%. In the embodiment of the present invention, the mass ratio of the added acidic solution to the lithium iron phosphate raw material is [3-5]:1, for example, 3:1, 3.5:1, 4:1, 4.5:1 or 5:1.

[0024] Step S2: adding ferrous sulfate solution to the first filtrate to obtain a mixed solution;

[0025] The amount of ferrous sulfate solution added is based on the iron-phosphorus molar ratio in the mixed solution satisfying: Fe / P=1.45-1.5.

[0026] Step S3: adding hydrogen peroxide and ammonia solution to the mixed solution to adjust the pH value, filtering and washing after the reaction to obtain a second filtrate and a filter cake, respectively, washing and drying the filter cake to obtain hydroxyferric phosphate;

[0027] Wherein, the concentration of the hydrogen peroxide solution can be 20-30wt%, for example, 20wt%, 25wt% or 30wt%; the concentration of the ammonia solution can be 20-30wt%, for example, 20wt%, 25wt% or 30wt%; the pH value is adjusted to 2.0-4.5 by adding hydrogen peroxide and ammonia solution, for example, the pH value can be adjusted to 2, 2.5, 3, 3.5, 4 or 4.5.

[0028] Step S4: adding barium hydroxide solution to the second filtrate, filtering after the reaction is complete to obtain a third filtrate;

[0029] Wherein, the barium hydroxide solution is added to remove sulfate ions in the second filtrate.

[0030] Step S5: adding a phosphoric acid solution to the third filtrate to obtain a fourth filtrate;

[0031] The amount of phosphoric acid solution added is based on the lithium-phosphorus molar ratio in the fourth filtrate satisfying: Li / P=2.8-3.2, for example, Li / P=2.8, Li / P=2.9, Li / P=3, Li / P=3.1 or Li / P=3.2.

[0032] Step S6: mixing the hydroxyferric phosphate, the fourth filtrate and the carbon source, sand-grinding the slurry, and drying to obtain a powder;

[0033] Wherein, after the hydroxy ferric phosphate and the fourth filtrate are mixed, the molar ratio of iron element to phosphorus element satisfies n(Fe):n(Li)=1:[1-1.04], for example, n(Fe):n(Li)=1:1, n(Fe):n(Li)=1:1.01, n(Fe):n(Li)=1:1.02, n(Fe):n(Li)=1:1.03 or n(Fe):n(Li)=1:1.04; the amount of the carbon source added is based on the carbon content in the lithium iron phosphate material between 0.5wt% and 3wt%. Preferably, the amount of the carbon source added is based on the lithium iron phosphate material. The carbon content in the material is preferably between 0.8-2wt%, for example, the carbon content can be 0.8wt%, 1.0wt%, 1.2wt%, 1.5wt%, 1.8wt% or 2.0wt%; in an embodiment of the present invention, the carbon source is a mixture of glucose and polyethylene glycol; the slurry particle size is 0.40-0.95μm, preferably, the slurry particle size is 0.50-0.75μm, for example, the slurry particle size is 0.50μm, 0.55μm, 0.60μm, 0.65μm, 0.70μm or 0.75μm; the powder particle size is 20-40μm.

[0034] Step S7: sintering the powder in an inert atmosphere, and crushing it after natural cooling to obtain a carbon-coated lithium iron phosphate material.

[0035] The gas used in the inert atmosphere is selected from at least one of nitrogen, argon and helium; the sintering temperature can be 750-900°C, for example, the sintering temperature can be 750°C, 760°C, 770°C, 780°C, 790°C, 800°C, 810°C, 820°C, 830°C, 840°C, 850°C, 860°C, 870°C, 880°C, 890°C or 900°C; the sintering time can be The sintering time is 5-12h, for example, the sintering time can be 5h, 6h, 7h, 8h, 9h, 10h, 11h or 12h; the particle size of the carbon-coated lithium iron phosphate material is 0.80-3μm, preferably, the particle size of the carbon-coated lithium iron phosphate material is 1-2μm, for example, the particle size of the carbon-coated lithium iron phosphate material is 1μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm or 2μm.

[0036] The embodiment of the present invention provides a method for recycling waste lithium iron phosphate materials to prepare lithium iron phosphate. When the waste lithium iron phosphate materials are processed, the iron element and the phosphorus element are recovered in the form of hydroxy iron phosphate by adjusting the amount of iron source added and the pH value, so that the recovery rates of the iron element and the phosphorus element are as high as 94.3% and 94.2% or more, respectively, and the amount of lithium element precipitated when the phosphorus element and the iron element are precipitated is minimized, and the lithium element is separated from the phosphorus element and the iron element. The method provided by the present invention recovers the phosphorus element and the iron element in the form of hydroxy iron phosphate and recovers the lithium element in the form of lithium hydroxide, thereby improving the recovery rates of the lithium element, the phosphorus element and the iron element.

[0037] When preparing lithium iron phosphate material later, adding phosphoric acid to the filtrate can not only adjust the Li / P ratio to an appropriate range, but also prevent the precipitation of lithium elements, so that the lithium elements can be recovered in the filtrate in the form of lithium hydroxide. The lithium recovery rate can be as high as 92.8% or more.

[0038] Moreover, the nano-scale lithium iron phosphate material prepared by the method provided by the present invention has uniform particle size and excellent material performance: the 0.1C first discharge capacity can reach more than 162mAh / g, and the 1C discharge capacity can reach 142mAh / g. In addition, the method has high production efficiency, low production cost, and is environmentally friendly, suitable for large-scale industrial production, and has high economic and social benefits.

[0039] The specific process and effect of the method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials of the present invention are further described in detail below in combination with some specific embodiments, but are not limited to the protection scope of the present invention.

[0040] Example 1

[0041] This embodiment provides a method for recycling waste lithium iron phosphate materials to prepare lithium iron phosphate, comprising the following steps:

[0042] Step S1: crushing the waste lithium iron phosphate battery positive electrode sheet to a particle size of 4-6 μm, oscillating and sieving to obtain a lithium iron phosphate raw material, adding a sulfuric acid solution with a concentration of 30 wt % to the lithium iron phosphate raw material, wherein the mass ratio of the sulfuric acid solution to the lithium iron phosphate raw material is 4:1, filtering and collecting a first filtrate;

[0043] Step S2: adding ferrous sulfate solution to the first filtrate, adjusting the Fe / P molar ratio to 1.45, to obtain a mixed solution;

[0044] Step S3: adding 25 wt % hydrogen peroxide and 25 wt % ammonia solution to the mixed solution to adjust the pH value to 2.5, filtering and washing after the reaction to obtain a second filtrate and a filter cake, respectively, washing and drying the filter cake to obtain hydroxyferric phosphate;

[0045] Step S4: adding barium hydroxide solution to the second filtrate to precipitate sulfate ions, filtering after the reaction is complete, and washing to obtain a third filtrate;

[0046] Step S5: adding a phosphoric acid solution to the third filtrate to adjust the Li / P molar ratio to 3 to obtain a fourth filtrate;

[0047] Step S6: uniformly mixing the ferric hydroxyphosphate, the fourth filtrate, glucose and polyethylene glycol, wherein the molar ratio of iron to phosphorus is 1:1.04, the carbon content is 1.10 wt %, and the slurry is sand-ground to a particle size of 0.65 μm, and fully dried to obtain a powder;

[0048] Step S7: The powder is kept at 780° C. for 10 hours in a nitrogen atmosphere, and then naturally cooled and crushed to a particle size of 1.2 μm to obtain a carbon-coated nano-scale lithium iron phosphate material.

[0049] in, Figure 3 It is the SEM spectrum of the lithium iron phosphate material prepared in this example.

[0050] Example 2

[0051] This embodiment provides a method for recycling waste lithium iron phosphate materials to prepare lithium iron phosphate, comprising the following steps:

[0052] Step S1: crushing the waste lithium iron phosphate battery positive electrode sheet to a particle size of 7-9 μm, oscillating and sieving to obtain a lithium iron phosphate raw material, adding a sulfuric acid solution with a concentration of 30 wt % to the lithium iron phosphate raw material, wherein the mass ratio of the sulfuric acid solution to the lithium iron phosphate raw material is 4.5:1, filtering and collecting a first filtrate;

[0053] Step S2: adding ferrous sulfate solution to the first filtrate, adjusting the Fe / P molar ratio to 1.47, to obtain a mixed solution;

[0054] Step S3: adding 25 wt % hydrogen peroxide and 25 wt % ammonia solution to the mixed solution to adjust the pH value to 3.0, filtering and washing after the reaction to obtain a second filtrate and a filter cake, respectively, washing and drying the filter cake to obtain hydroxyferric phosphate;

[0055] Step S4: adding barium hydroxide solution to the second filtrate to precipitate sulfate ions, filtering after the reaction is complete, and washing to obtain a third filtrate;

[0056] Step S5: adding a phosphoric acid solution to the third filtrate to adjust the Li / P molar ratio to 2.9 to obtain a fourth filtrate;

[0057] Step S6: uniformly mixing the ferric hydroxyphosphate, the fourth filtrate, glucose and polyethylene glycol, wherein the molar ratio of iron to phosphorus is 1:1.03, the carbon content is 1.30 wt %, and the slurry is sand-ground to a particle size of 0.55 μm, and fully dried to obtain a powder;

[0058] Step S7: The powder is kept at 820° C. for 12 h in a nitrogen atmosphere, and then naturally cooled and crushed to a particle size of 0.95 μm to obtain a carbon-coated nano-scale lithium iron phosphate material.

[0059] Comparative Example 1

[0060] This embodiment provides a method for recycling waste lithium iron phosphate materials to prepare lithium iron phosphate, comprising the following steps:

[0061] Step S1: crushing the waste lithium iron phosphate battery positive electrode sheet to a particle size of 4-6 μm, oscillating and sieving to obtain a lithium iron phosphate raw material, adding a sulfuric acid solution with a concentration of 30 wt % to the lithium iron phosphate raw material, wherein the mass ratio of the sulfuric acid solution to the lithium iron phosphate raw material is 5:1, filtering and collecting a first filtrate;

[0062] Step S2: adding ferrous sulfate solution to the first filtrate, adjusting the Fe / P molar ratio to 1.45, to obtain a mixed solution;

[0063] Step S3: adding 25 wt % hydrogen peroxide and 25 wt % ammonia solution to the mixed solution to adjust the pH value to 1.0, filtering and washing after the reaction to obtain a second filtrate and a filter cake, respectively, washing and drying the filter cake to obtain hydroxyferric phosphate;

[0064] Step S4: adding barium hydroxide solution to the second filtrate to precipitate sulfate ions, filtering after the reaction is complete, and washing to obtain a third filtrate;

[0065] Step S5: adding a phosphoric acid solution to the third filtrate to adjust the Li / P molar ratio to 3 to obtain a fourth filtrate;

[0066] Step S6: uniformly mixing the ferric hydroxyphosphate, the fourth filtrate, glucose and polyethylene glycol, wherein the molar ratio of iron to phosphorus is 1:1.04, the carbon content is 1.50 wt %, and the slurry is sand-ground to a particle size of 0.85 μm, and fully dried to obtain a powder;

[0067] Step S7: The powder is kept at 780° C. for 5 h in a nitrogen atmosphere, and then naturally cooled and crushed to a particle size of 1.0 μm to obtain a carbon-coated nano-scale lithium iron phosphate material.

[0068] In order to verify the quality of the finished lithium iron phosphate positive electrode material prepared by the method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials provided by the embodiment of the present invention, the lithium iron phosphate positive electrode material prepared in the above-mentioned embodiment 1-2 and comparative example 1, the conductive agent carbon black and the binder polyvinylidene fluoride are dispersed in N-methylpyrrolidone according to a mass ratio of 80:10:10, and after being evenly dispersed by ball milling, they are coated on aluminum foil and vacuum dried to obtain a positive electrode sheet. The electrolyte is 1 mol / L LiPF6, wherein the solvent volume ratio is EC:DMC:EMC=1:1:1, the diaphragm is Celgard polypropylene film, and the metal lithium sheet is the negative electrode, which are assembled together into a button half-cell. The test voltage range is 2.0V-3.75V, and it is charged to 3.75V in a constant current and constant voltage charging mode, and discharged to 2.0V in a constant current discharge mode. The charge and discharge current is 0.1C per cycle, and then the charge and discharge current is 1C per cycle. The cut-off condition is the same as 0.1C, and the test results are shown in Table 1.

[0069] Table 1. Test items and test results of Examples 1-2 and Comparative Example 1

[0070]

[0071]

[0072] According to the above embodiments and control examples and the comparison of the test results obtained therefrom, the button-type half-cell prepared with the lithium iron phosphate positive electrode material in Embodiments 1-2 has significantly improved 0.1C first charge and discharge specific capacity and 1C discharge specific capacity compared with Comparative Example 1.

[0073] In summary, the method for recycling waste lithium iron phosphate materials to prepare lithium iron phosphate provided in an embodiment of the present invention, when processing waste lithium iron phosphate materials, by adjusting the amount of iron source added and the pH value, the iron element and the phosphorus element are recovered in the form of hydroxy iron phosphate, so that the recovery rates of iron element and phosphorus element are as high as 94.3% and 94.2% or more, respectively, and the amount of lithium element precipitated when phosphorus element and iron element are precipitated is minimized, and the lithium element is separated from the phosphorus element and the iron element; the method provided by the present invention recovers the phosphorus element and the iron element in the form of hydroxy iron phosphate and recovers the lithium element in the form of lithium hydroxide, thereby improving the recovery rates of lithium element, phosphorus element and iron element.

[0074] When preparing lithium iron phosphate material later, adding phosphoric acid to the filtrate can not only adjust the Li / P ratio to an appropriate range, but also prevent the precipitation of lithium elements, so that the lithium elements can be recovered in the filtrate in the form of lithium hydroxide. The lithium recovery rate can be as high as 92.8% or more.

[0075] Moreover, the nano-scale lithium iron phosphate material prepared by the method provided by the present invention has uniform particle size and excellent material performance: the 0.1C first discharge capacity can reach more than 162mAh / g, and the 1C discharge capacity can reach 142mAh / g. In addition, the method has high production efficiency, low production cost, and is environmentally friendly, suitable for large-scale industrial production, and has high economic and social benefits.

[0076] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example" or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, unless they are contradictory.

[0077] Although embodiments of the present invention have been shown and described, those skilled in the art will appreciate that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the claims and their equivalents.

Claims

1. A method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials, characterized in that: The method comprises: Step S1: crushing the waste lithium iron phosphate battery positive electrode sheet, shaking and sieving to obtain a lithium iron phosphate raw material, dissolving the lithium iron phosphate raw material in an acidic solution, filtering and collecting a first filtrate; Step S2: adding ferrous sulfate solution to the first filtrate to obtain a mixed solution; Step S3: adding hydrogen peroxide and ammonia solution to the mixed solution to adjust the pH value, filtering and washing after the reaction to obtain a second filtrate and a filter cake, respectively, washing and drying the filter cake to obtain hydroxyferric phosphate; Step S4: adding barium hydroxide solution to the second filtrate, filtering after the reaction is complete to obtain a third filtrate; Step S5: adding a phosphoric acid solution to the third filtrate to obtain a fourth filtrate; Step S6: mixing the hydroxyferric phosphate, the fourth filtrate and the carbon source, sand-grinding the slurry, and drying to obtain a powder; Step S7: sintering the powder in an inert atmosphere, and crushing it after natural cooling to obtain a carbon-coated lithium iron phosphate material.

2. The method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials according to claim 1, characterized in that: In the step S1, the acidic solution is a sulfuric acid solution or a phosphoric acid solution, and the concentration of the acidic solution is greater than 30 wt%.

3. The method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials according to claim 1, characterized in that: In step S1, the mass ratio of the added acidic solution to the lithium iron phosphate raw material is [3-5]:

1.

4. The method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials according to claim 1, characterized in that: In step S2, the amount of ferrous sulfate solution added is based on the iron-phosphorus molar ratio in the mixed solution satisfying: Fe / P=1.45-1.

5.

5. The method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials according to claim 1, characterized in that: In step S3, the concentration of the hydrogen peroxide solution is 20-30 wt %, the concentration of the ammonia solution is 20-30 wt %, and the hydrogen peroxide solution and the ammonia solution are added to adjust the pH value to 2.0-4.

5.

6. The method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials according to claim 1, characterized in that: In the step S4, a barium hydroxide solution is added to remove sulfate ions in the second filtrate.

7. The method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials according to claim 1, characterized in that: In the step S5, the amount of phosphoric acid solution added is based on the lithium-phosphorus molar ratio in the fourth filtrate satisfying: Li / P=2.8-3.

2.

8. The method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials according to claim 1, characterized in that: In the step S6, after the hydroxyferric phosphate and the fourth filtrate are mixed, the molar ratio of iron element to phosphorus element satisfies n(Fe):n(Li)=1:[1-1.04]; the amount of the carbon source added is based on the carbon content in the lithium iron phosphate material between 0.5-3wt%, and the carbon source is a mixture of glucose and polyethylene glycol; the slurry particle size is 0.40-0.95μm.

9. The method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials according to claim 8, characterized in that: In the step S6, the amount of the carbon source added is based on the carbon content in the lithium iron phosphate material being between 0.8-2 wt%, the slurry particle size is 0.50-0.75 μm, and the powder particle size is 20-40 μm.

10. The method for preparing lithium iron phosphate by recycling waste lithium iron phosphate materials according to claim 1, characterized in that: In step S7, the gas used in the inert atmosphere is selected from at least one of nitrogen, argon and helium; the sintering temperature is 750-900° C., and the sintering time is 5-12 hours; and the particle size of the carbon-coated lithium iron phosphate material is 0.80-3.0 μm.

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