Method for regenerating waste lithium iron phosphate battery into lithium manganese iron phosphate positive electrode material

Abstract

This invention discloses a method for regenerating waste lithium iron phosphate batteries into lithium manganese iron phosphate cathode material, comprising the following steps: obtaining lithium sulfate solution and iron phosphate slag by oxidative acid leaching of lithium iron phosphate battery powder; calcining a mixture of manganese oxide and phosphoric acid slag to obtain ferric oxide doped with Mn and phosphorus pentoxide gas; collecting the phosphorus pentoxide gas to obtain a phosphoric acid solution, and reacting the phosphoric acid solution with ammonia and lithium sulfate solution to obtain lithium phosphate; reacting the Mn-doped ferric oxide with sulfur dioxide to obtain ferrous sulfate doped with Mn; and reacting the lithium phosphate and ferrous sulfate doped with Mn with water to obtain lithium manganese iron phosphate cathode material. The regeneration method of this invention can effectively separate lithium / iron-phosphorus, while effectively improving the recovery rate of phosphorus and iron (phosphorus recovery rate > 99%, iron recovery rate > 99%), to directly synthesize high-purity battery-grade lithium manganese iron phosphate cathode material. This method breaks through the traditional long process of dissolution-separation-resynthesis, greatly shortening the process chain and reducing material loss and energy consumption.

Description

Technical Field

[0001] This invention belongs to the field of waste lithium iron phosphate battery recycling technology, and particularly relates to a method for regenerating waste lithium iron phosphate batteries into lithium manganese iron phosphate cathode material. Background Technology

[0002] The recycling of spent lithium iron phosphate batteries currently falls into two main technical paths: dismantling and secondary use, and recycling and material regeneration. In the field of material regeneration, wet recycling processes are commonly used. This typically involves leaching the battery materials using a sulfuric acid and hydrogen peroxide system, followed by complex chemical precipitation, separation, and purification processes to extract elements such as lithium, iron, and phosphorus, ultimately resynthesizing lithium iron phosphate. However, this technology suffers from drawbacks such as cumbersome processes, high hydrogen peroxide costs, and significant environmental impact. Furthermore, the final products are often limited to intermediates like lithium carbonate and iron phosphate. In contrast, research on high-temperature calcination technology focuses primarily on the direct regeneration of lithium iron phosphate cathode materials. This process involves mechanical crushing → high-temperature calcination → reduction smelting → metal separation to obtain the battery raw materials. During this process, the recovered materials are primarily iron and manganese oxides.

[0003] Whether using high-temperature calcination technology or wet recycling processes, it is necessary to first separate and extract iron, phosphorus, and lithium from waste battery materials. Generally speaking, the higher the leaching rate of lithium, iron, and phosphorus, the higher the yield of recovered lithium iron phosphate. However, both existing wet recycling and high-temperature calcination technologies result in either low phosphorus-iron recovery rates or low lithium recovery rates, leading to insufficient recovery of both phosphorus and iron / lithium.

[0004] Therefore, we are developing a method based on waste lithium iron phosphate batteries as raw materials, which can separate lithium / iron-phosphorus, with a focus on effectively improving the recovery rate of iron and phosphorus, and at the same time regenerating them into lithium battery cathode materials, thereby achieving cost reduction and efficiency improvement and enhancing industrial application. Summary of the Invention

[0005] Purpose of the invention: The technical problem to be solved by the present invention is to provide a method for regenerating lithium battery cathode materials from waste lithium iron phosphate batteries, so as to separate lithium / iron phosphorus and effectively improve the recovery rate of iron and phosphorus, thereby reducing costs and increasing efficiency and improving industrial application.

[0006] Technical solution: The present invention provides a method for regenerating spent lithium iron phosphate batteries into lithium manganese iron phosphate cathode material, comprising the following steps:

[0007] (1) Obtain crude lithium sulfate solution and iron phosphate slag by oxidative acid leaching of lithium iron phosphate battery powder, and purify the crude lithium sulfate solution to obtain refined lithium sulfate solution;

[0008] (2) After mixing manganese oxide with phosphoric acid residue, the mixture is first roasted at 400-600℃ under oxygen-enriched conditions for 1-3 hours, and then roasted at 800-1050℃ for 4-8 hours to obtain Mn-doped ferric oxide and phosphorus pentoxide gas; wherein the amount of manganese oxide added is 0.15-0.25 times the number of moles of iron in the ferric phosphate residue;

[0009] (3) Collect phosphorus pentoxide gas to obtain phosphoric acid solution, and then ammonify the phosphoric acid solution and react it with the purified lithium sulfate solution from step (1) to obtain lithium phosphate;

[0010] (4) React the Mn-doped ferric oxide obtained in step (2) with sulfur dioxide to obtain Mn-doped ferrous sulfate;

[0011] (5) The lithium phosphate from step (3) and the Mn-doped ferrous sulfate from step (4) are mixed with water and reacted at 110-120°C under an inert atmosphere for 6-8 hours to generate lithium manganese iron phosphate cathode material.

[0012] This invention, based on the method of obtaining refined lithium sulfate through oxidative roasting and purification, specifically targets iron-phosphorus slag. By introducing manganese oxide into the slag mixture and calcining at high temperature, the solid manganese oxide provides active oxygen (its oxidation potential is much higher than that of gaseous oxygen) during the roasting process. This active oxygen directly acts on the solid-phase interface, directly attacking and breaking the high-bonding-energy Fe-P bond, thus deeply disrupting the iron-phosphorus lattice. This lowers the activation energy of the oxidative dephosphorization reaction, making it easier for phosphorus to be oxidized into P2O5 gas and escape, effectively separating iron and phosphorus. Simultaneously, free manganese ions (with a +3 valence and an ionic radius close to that of iron ions) can enter the lattice positions of iron oxide, replacing some iron atoms to form Mn-doped iron oxide.

[0013] Furthermore, the present invention employs a two-step roasting method. First, under low-temperature conditions, without melting the material, the easily reactive phosphorus is discharged as phosphorus pentoxide gas, thereby achieving the purpose of creating pores. This forms an exhaust channel for the phosphorus pentoxide gas formed during high-temperature roasting, which helps to improve the separation of iron and phosphorus.

[0014] Furthermore, the manganese-containing ferric oxide precipitate obtained from calcination reacts with sulfur dioxide in subsequent steps to generate Mn-doped FeSO4; and this Mn-doped FeSO4 further reacts with lithium phosphate, producing Mn... 2+ It will preferentially adsorb the negatively charged PO4 on the surface of Li3PO4. 3- At the site, a manganese-rich pre-reaction layer is formed; the Mn in this pre-reaction layer 2+ With the remaining Mn in the solution 2+ / Fe 2+ There is an affinity between the same elements (Mn) 2+ -Mn2+ Interactions) reduce the interfacial energy, subsequently affecting Fe 2+ The insertion resistance is reduced, thereby improving the synthesis efficiency of lithium manganese iron phosphate cathode materials.

[0015] Furthermore, in step (1) of the preparation method, the oxidative acid leaching is carried out by mixing lithium iron phosphate battery powder with water at a liquid-solid ratio of (2-4):1, adding sulfuric acid, and introducing oxygen until the pressure reaches 0.3-1.0 MPa, and then reacting at 60-80℃ for 2-6 hours.

[0016] Furthermore, in step (1) of this preparation method, the purification step of the crude lithium sulfate solution is as follows:

[0017] 1) Add an oxidizing agent to the crude lithium sulfate solution to make Fe 2+ Oxidized to Fe 3+ A pretreated lithium sulfate solution is obtained;

[0018] 2) Adjust the pH of the pretreated lithium sulfate solution to 5.0-6.0 with alkali, stir and react at 50-60℃ for 30-60 min, and filter to obtain purified lithium solution;

[0019] 3) Add sodium sulfide solution to the purified lithium solution, control the pH of the system to 6.0-7.0, stir the reaction for 20-30 minutes to obtain a refined lithium sulfate solution, and then concentrate it to obtain a lithium sulfate solution of 20-25 g / L.

[0020] Furthermore, in step (3) of the preparation method, the ammoniation of the phosphoric acid solution involves adding ammonia water dropwise to the phosphoric acid solution to adjust its pH value to 9-10.

[0021] Furthermore, in step (3) of the preparation method, the reaction of the ammonified phosphoric acid solution with the refined lithium sulfate solution in step (1) has a lithium to phosphorus molar ratio of (2.9-3.2):1, a reaction condition of 60-80℃, and a reaction time of 2-4h.

[0022] Furthermore, in step (4) of this preparation method, the reaction between the Mn-doped ferric oxide and sulfur dioxide is as follows:

[0023] 1) Mix Mn-doped ferric oxide with dilute sulfuric acid to form a slurry;

[0024] 2) Heat the slurry to 60-90℃ and introduce sulfur dioxide gas to reduce it until the ferric iron is completely converted to ferrous iron. Then stop the gas introduction and filter to obtain the filtrate.

[0025] 3) Concentrate the filtrate under reduced pressure, cool to crystallize, separate and dry to obtain Mn-doped ferrous sulfate crystals.

[0026] Furthermore, in step (4) of the preparation method, the concentration of the dilute sulfuric acid is 0.5-2 mol / L, the solid-liquid ratio of the slurry is 1:(5-10), the flow rate of sulfur dioxide is 30-150 mL / min, the concentration temperature is 50-70℃, and the crystallization temperature is 0-5℃.

[0027] Furthermore, in step (5) of the preparation method, the molar ratio of Li:Fe:P in the lithium phosphate and Mn-doped ferrous sulfate is (0.98-1.05):1:(0.98-1.05).

[0028] Furthermore, in step (5) of the preparation method, the amount of water added is such that the solid content is 10-25%.

[0029] Beneficial effects: Compared with the prior art, the significant advantages of this invention are: the regeneration method can effectively separate lithium / iron phosphorus and effectively improve the recovery rate of phosphorus and iron (phosphorus recovery rate > 99%, iron recovery rate > 99%) to directly synthesize high-purity battery-grade lithium manganese iron phosphate cathode material, breaking through the traditional long process mode of dissolution-separation-resynthesis, greatly shortening the process chain, and reducing material loss and energy consumption. Detailed Implementation

[0030] The technical solution of the present invention will be further described in detail below with reference to the embodiments.

[0031] In the embodiments and comparative examples of this invention, the amount of sulfuric acid required for oxidative acid leaching is one times the theoretical amount of sulfuric acid required for leaching Li / Fe / Cu / Ni / Co / Cu / Al. Furthermore, the lithium iron phosphate battery powder is obtained from waste lithium iron phosphate batteries through discharge, dismantling, and crushing.

[0032] Example 1

[0033] The composition and content of the waste lithium iron phosphate battery powder in Example 1 are shown in Table 1.

[0034] Table 1 Main Components of Lithium Iron Phosphate Batteries

[0035]

[0036] This embodiment 1 describes a method for regenerating lithium manganese iron phosphate cathode material from spent lithium iron phosphate batteries, comprising the following steps:

[0037] (1) Take 100g of treated waste battery powder and mix it with water at a liquid-solid ratio of 2:1. After stirring for 30 minutes, slowly add 32.14g of sulfuric acid with a mass fraction of 98%. Then add water to make the liquid-solid ratio 3:1. Introduce oxygen to the initial pressure of 0.5 MPa. Stir and react at 70℃ for 4 hours. Then perform hot filtration to obtain 92.46g of phosphorus iron slag (deep yellow filter residue) and crude lithium sulfate solution (light green filtrate).

[0038] (2) The crude lithium sulfate solution is purified to obtain a refined lithium sulfate solution. The specific steps are as follows:

[0039] 1) Add 30% hydrogen peroxide to the crude lithium sulfate solution, the amount of which is such that all Fe²⁺ is oxidized to Fe³⁺, to obtain a pretreated lithium sulfate solution;

[0040] 2) The pH of the lithium-containing solution was adjusted to 5.5 using sodium hydroxide solution, heated to 60°C, and stirred for 1 hour to generate a large amount of reddish-brown flocculent precipitate. The precipitate was then filtered to obtain purified lithium solution.

[0041] 3) Add 5.61g of Na2S solid to the purified lithium solution, control the pH of the system to 6.0-7.0, stir and react for 30min, then filter to obtain a colorless and transparent purified lithium sulfate solution; and concentrate by MVR evaporation to finally obtain 135mL of purified lithium sulfate solution with a concentration of 20 g / L.

[0042] (3) The amount of manganese oxide added is 0.2 times the number of iron moles in the phosphorus iron slag in step (1), i.e. 5.29g of manganese oxide. After mixing manganese oxide with phosphorus iron slag, it is first roasted at 500℃ under oxygen-rich conditions for 2h, and then roasted at 950℃ for 6h. During this period, the gas generated by roasting is continuously introduced into the water absorption device to obtain a colorless and transparent phosphoric acid solution and ferric oxide doped with Mn.

[0043] (4) After ammoniation of the phosphoric acid solution, it is reacted with the purified lithium sulfate solution to generate lithium phosphate. The specific steps are as follows:

[0044] 1) Cool the phosphoric acid solution and slowly add concentrated ammonia dropwise while stirring until the pH reaches about 9.5 to obtain an ammonium phosphate solution;

[0045] 2) Under stirring, the ammonium phosphate solution was slowly added to the concentrated lithium sulfate solution (controlling n(Li):n(P) = 3:1). A large amount of white precipitate was generated. After aging at 70℃ for 3 hours, the precipitate was filtered, washed with hot water, and dried at 80℃ for 12 hours to obtain white powdered lithium phosphate.

[0046] (5) Based on Mn-doped ferric oxide, it is reacted with sulfur dioxide to obtain Mn-doped ferrous sulfate. The specific steps are as follows:

[0047] 1) Mix Mn-doped ferric oxide with 1 mol / L dilute sulfuric acid to form a slurry with a solid-liquid ratio of 1:8;

[0048] 2) Heat the slurry to about 80°C and introduce sulfur dioxide gas at a flow rate of 80 mL / min to completely convert ferric iron to ferrous iron. Then stop the gas introduction and filter to obtain the filtrate.

[0049] 3) The filtrate was concentrated under reduced pressure at 60°C, then cooled and crystallized at 2°C. The crystals were separated and dried to obtain Mn-doped ferrous sulfate crystals.

[0050] (6) Mix Mn-doped ferrous sulfate crystals, lithium phosphate and deionized water in a stoichiometric ratio (Li:Fe:P = 1:1:1) to prepare a uniform slurry with a solid content of 20%. Transfer the slurry into a high-pressure reactor and react at 115°C under an inert atmosphere for 7 hours. After the reaction is completed, the product is centrifuged, washed and dried at 85°C to obtain 56.41 g of yellowish-brown lithium manganese iron phosphate powder.

[0051] The molar ratio of the main elements in the ICP was 1:0.9:1 (Li, Fe, P molar ratio). The Fe / P recovery rate in lithium manganese iron phosphate was calculated as follows:

[0052] Fe=56.41×0.3381 / 19.23×100=99.2%; P=56.41×0.2075 / 11.74×100=99.7%.

[0053] Comparative Example 1

[0054] The basic steps are the same as in Example 1, except that in step (3), the phosphorus iron slag is first roasted at 500°C under oxygen-enriched conditions for 2 hours, and then roasted at 950°C for 6 hours to obtain 52.25g of lithium manganese iron phosphate. The Fe / P recovery rate of this lithium manganese iron phosphate is:

[0055] Fe=52.25×0.3121 / 19.23×100=84.8%; P=52.25×0.1955 / 11.74×100=86.9%.

[0056] Example 2

[0057] The composition and content of the waste lithium iron phosphate battery powder in Example 2 are shown in Table 2.

[0058] Table 2 Main Components of Lithium Iron Phosphate Batteries

[0059]

[0060] This embodiment 2, which uses waste lithium iron phosphate batteries as raw materials to regenerate lithium manganese iron phosphate cathode materials, is basically the same as that in embodiment 1, except for the main components of the waste lithium iron phosphate batteries and the process parameters of step (3). The details are as follows:

[0061] (1) Take 100g of treated waste battery powder and mix it with water at a liquid-solid ratio of 2:1. After stirring for 30 minutes, slowly add 23.01g of sulfuric acid with a mass fraction of 98%. Then add water to make the liquid-solid ratio 3:1. Introduce oxygen to the initial pressure of 0.5 MPa. Stir and react at 70℃ for 4 hours. Then perform hot filtration to obtain 94.3g of phosphorus iron slag (deep yellow filter residue) and crude lithium sulfate solution (light green filtrate).

[0062] (2) The crude lithium sulfate solution is purified to obtain a refined lithium sulfate solution. The specific steps are as follows:

[0063] 1) Add 30% hydrogen peroxide to the crude lithium sulfate solution, the amount of which is such that all Fe²⁺ is oxidized to Fe³⁺, to obtain a pretreated lithium sulfate solution;

[0064] 2) Adjust the pH of the lithium-containing solution to 5.5 with NaOH solution, heat to 60℃, stir for 1 hour to generate a large amount of reddish-brown flocculent precipitate, filter and obtain purified lithium solution;

[0065] 3) Add 1.4g of Na2S solid to the purified lithium solution, control the pH of the system to 6.0-7.0, stir and react for 30min, then filter to obtain a colorless and transparent purified lithium sulfate solution; and concentrate by MVR evaporation to finally obtain 140mL of purified lithium sulfate solution with a concentration of 20 g / L.

[0066] (3) The amount of manganese oxide added is 0.15 times the number of iron moles in the phosphorus iron slag in step (1), that is, the mass of manganese oxide is 4.98g. After mixing manganese oxide with phosphorus iron slag, it is first roasted at 450℃ under oxygen-enriched conditions for 3h, and then roasted at 1000℃ for 8h. During this period, the gas generated by roasting is continuously introduced into the water absorption device to obtain a colorless and transparent phosphoric acid solution and ferric oxide doped with Mn.

[0067] (4) After ammoniation of the phosphoric acid solution, it is reacted with the purified lithium sulfate solution to generate lithium phosphate. The specific steps are as follows:

[0068] 1) Cool the phosphoric acid solution and slowly add concentrated ammonia dropwise while stirring until the pH reaches about 9.5 to obtain an ammonium phosphate solution;

[0069] 2) Under stirring, the ammonium phosphate solution was slowly added to the concentrated lithium sulfate solution (controlling n(Li):n(P) = 3:1). A large amount of white precipitate was generated. After aging at 70°C for 3 hours, the precipitate was filtered, washed with hot water, and dried at 80°C for 12 hours to obtain white powdered lithium phosphate.

[0070] (5) Based on Mn-doped ferric oxide, it is reacted with sulfur dioxide to obtain Mn-doped ferrous sulfate. The specific steps are as follows:

[0071] 1) Mix Mn-doped ferric oxide with 1 mol / L dilute sulfuric acid to form a slurry with a solid-liquid ratio of 1:8;

[0072] 2) Heat the slurry to about 80°C and introduce sulfur dioxide gas at a flow rate of 80 mL / min to completely convert ferric iron to ferrous iron. Then stop the gas introduction and filter to obtain the filtrate.

[0073] 3) The filtrate was concentrated under reduced pressure at 60°C, then cooled and crystallized at 2°C. The crystals were separated and dried to obtain Mn-doped ferrous sulfate crystals.

[0074] (6) Ferrous sulfate, lithium phosphate and deionized water were mixed in stoichiometric ratio (Li:Fe:P = 1:1:1) to prepare a uniform slurry with a solid content of 20%. The slurry was transferred into an autoclave and reacted at 115°C for 7 hours. After the reaction was completed, the product was centrifuged, washed and dried at 85°C to obtain 66.5g of yellowish-brown lithium manganese iron phosphate powder.

[0075] The molar ratio of the main elements in the ICP was 1:0.9:1 (Li, Fe, P molar ratio). The Fe / P recovery rates in lithium manganese iron phosphate were calculated as follows: Fe = 66.5 × 0.3555 / 23.8 × 100 = 99.3%; P = 66.5 × 0.1871 / 12.52 × 100 = 99.4%.

[0076] Comparative Example 2

[0077] The basic steps are the same as in Example 2, except that in step (3), the phosphorus iron slag is first roasted at 450°C under oxygen-enriched conditions for 3 hours, and then roasted at 1000°C for 8 hours to obtain 62.5g of lithium manganese iron phosphate. The Fe / P recovery rate of this lithium manganese iron phosphate is:

[0078] Fe=62.5×0.3399 / 23.8×100=89.3%; P=62.5×0.1885 / 12.52×100=89.52%.

[0079] Performance testing

[0080] The purity of lithium manganese iron phosphate obtained from Examples 1 and 2, as well as Comparative Examples 1 and 2, was tested, and the results are shown in Table 3. The purity of the iron-containing product and the phosphorus removal rate in the examples and comparative examples are shown in Table 4, and their calculation methods are as follows:

[0081] Mn-doped Fe2O3 purity (%) = [Measured total (Fe+Mn) ÷ Theoretical total (Fe+Mn) of target product] × 100%;

[0082] Phosphorus removal rate (%) = [Total phosphorus in raw material - Residual phosphorus in iron-containing products] ÷ Total phosphorus in raw material × 100%.

[0083] Table 3 Purity of Lithium Manganese Iron Phosphate

[0084]

[0085] Table 4. Mass and purity of Mn-doped Fe2O3

[0086]

[0087] The process of this invention can effectively separate lithium / iron-phosphorus, and the method of combining manganese oxide and phosphorus-iron slag for high-temperature roasting can improve the phosphorus / iron separation rate (phosphorus removal rate > 99%, iron recovery rate > 99%). In contrast, in Comparative Examples 1-2, the ordinary roasting method is used, and the phosphorus-iron bond energy is high, making it difficult to fully separate phosphorus and iron. Therefore, the phosphorus removal rate is < 97%, which leads to a decrease in the phosphorus-iron recovery rate.

[0088] In addition to the above embodiments, the recycling process and parameters of the present invention can achieve the technical effects claimed above, so no further testing is required to verify them.

Claims

1. A method for regenerating spent lithium iron phosphate batteries into lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: (1) Obtain crude lithium sulfate solution and iron phosphate slag by oxidative acid leaching of lithium iron phosphate battery powder, and purify the crude lithium sulfate solution to obtain refined lithium sulfate solution; (2) After mixing manganese oxide with ferrophosphate slag, the mixture is first roasted at 400-600℃ under oxygen-enriched conditions for 1-3 hours, and then roasted at 800-1050℃ for 4-8 hours to obtain ferric oxide doped with Mn and phosphorus pentoxide gas; wherein the amount of manganese oxide added is 0.15-0.25 times the number of moles of iron in the ferrophosphate slag; (3) Collect phosphorus pentoxide gas to obtain phosphoric acid solution, and then ammonify the phosphoric acid solution and react it with the purified lithium sulfate solution from step (1) to obtain lithium phosphate; (4) React the Mn-doped ferric oxide obtained in step (2) with sulfur dioxide to obtain Mn-doped ferrous sulfate; (5) The lithium phosphate from step (3) and the Mn-doped ferrous sulfate from step (4) are mixed with water and reacted at 110-120°C under an inert atmosphere for 6-8 hours to generate lithium manganese iron phosphate cathode material.

2. The method for regenerating spent lithium iron phosphate batteries into lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (1), the oxidative acid leaching is carried out by mixing lithium iron phosphate battery powder with water at a liquid-solid ratio of (2-4):1, adding sulfuric acid, and introducing oxygen until the pressure reaches 0.3-1.0 MPa, and then reacting at 60-80℃ for 2-6 hours.

3. The method for regenerating spent lithium iron phosphate batteries into lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (1), the purification step of the crude lithium sulfate solution is as follows: 1) Add an oxidizing agent to the crude lithium sulfate solution to make Fe 2+ Oxidized to Fe 3+ A pretreated lithium sulfate solution is obtained; 2) Adjust the pH of the pretreated lithium sulfate solution to 5.0-6.0 with alkali, stir and react at 50-60℃ for 30-60 min, and filter to obtain purified lithium solution; 3) Add sodium sulfide solution to the purified lithium solution, control the pH of the system to 6.0-7.0, stir the reaction for 20-30 minutes to obtain a refined lithium sulfate solution, and then concentrate it to obtain a lithium sulfate solution of 20-25 g / L.

4. The method for regenerating spent lithium iron phosphate batteries into lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (3), the ammoniation of the phosphoric acid solution involves adding ammonia water dropwise to the phosphoric acid solution to adjust its pH value to 9-10.

5. The method for regenerating spent lithium iron phosphate batteries into lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (3), the reaction of the ammonified phosphoric acid solution with the refined lithium sulfate solution in step (1) has a lithium to phosphorus molar ratio of (2.9-3.2):1, a reaction temperature of 60-80℃, and a reaction time of 2-4h.

6. The method for regenerating spent lithium iron phosphate batteries into lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (4), the reaction between the Mn-doped ferric oxide and sulfur dioxide is as follows: 1) Mix Mn-doped ferric oxide with dilute sulfuric acid to form a slurry; 2) Heat the slurry to 60-90℃ and introduce sulfur dioxide gas to reduce it until the ferric iron is completely converted to ferrous iron. Then stop the gas introduction and filter to obtain the filtrate. 3) Concentrate the filtrate under reduced pressure, cool to crystallize, separate and dry to obtain Mn-doped ferrous sulfate crystals.

7. The method for regenerating spent lithium iron phosphate batteries into lithium manganese iron phosphate cathode material according to claim 6, characterized in that, The concentration of the dilute sulfuric acid is 0.5-2 mol / L, the solid-liquid ratio of the slurry is 1:(5-10), the flow rate of sulfur dioxide is 30-150 mL / min, the concentration temperature is 50-70℃, and the crystallization temperature is 0-5℃.

8. The method for regenerating spent lithium iron phosphate batteries into lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step (5), the molar ratio of Li:Fe:P in the lithium phosphate and Mn-doped ferrous sulfate is (0.98-1.05):1:(0.98-1.05).

9. The method for regenerating spent lithium iron phosphate batteries into lithium manganese iron phosphate cathode material according to claim 2, characterized in that, In step (5), the amount of water added is such that the solid content is 10-25%.

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