A low-temperature hydrothermal regeneration method for waste lithium iron phosphate and application thereof and lithium ion battery

Abstract

The application provides a low-temperature hydrothermal regeneration method of waste lithium iron phosphate and application and a lithium ion battery, and relates to the technical field of lithium ion battery recycling and resource utilization. The low-temperature hydrothermal regeneration method comprises the following steps: mixing waste lithium iron phosphate powder and lithium salt, adding oleylamine and stirring to obtain a slurry precursor; performing hydrothermal reaction on the slurry precursor, cooling, obtaining a black suspension, centrifuging, discarding the supernatant and obtaining a solid product; and cleaning and drying the solid product to obtain regenerated lithium iron phosphate. The regeneration process effectively repairs key structural defects such as lithium deficiency, high content of trivalent iron (Fe³ + ) and Li / Fe inversion sites in the waste LFP, thereby endowing the regenerated LFP with excellent comprehensive electrochemical performance. The regeneration process is mild and simple, and is suitable for large-scale industrial application.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery recycling and resource utilization technology, and in particular to a low-temperature hydrothermal regeneration method for waste lithium iron phosphate and its application, as well as lithium-ion batteries. Background Technology

[0002] Lithium iron phosphate (LFP) batteries have become the mainstream choice in the power and energy storage fields due to their high safety, excellent cycle performance, and cost advantages. LFP batteries, with their high safety, long cycle life, and excellent high-temperature resistance, occupy a core position in marine energy storage. Through special structural design and protection technologies, LFP batteries can effectively achieve widespread application from the surface to the deep sea. However, LFP cathodes experience performance degradation after long-term use, mainly manifested as capacity reduction and decreased rate performance. The root cause is the loss of active lithium and the resulting Fe / Li anti-site defects. Facing the upcoming large-scale battery retirement wave, the development of efficient recycling technologies is crucial.

[0003] Lithium battery recycling mainly targets cathode materials. Currently, the mainstream recycling technologies for waste lithium iron phosphate (LiFePO4) cathode materials include hydrometallurgical methods, pyrometallurgical methods, and direct regeneration methods.

[0004] Hydrometallurgical processes typically involve leaching, impurity removal, and purification to recover metal elements. The leaching step often employs strong acid systems such as sulfuric acid and hydrochloric acid. While this method can achieve high metal leaching rates, it has significant drawbacks: the highly corrosive reagents place stringent requirements on equipment materials, easily causing equipment damage; furthermore, the process is lengthy and generates large amounts of acidic wastewater and neutralization slag containing heavy metal ions, resulting in high costs and significant pressure for subsequent environmental treatment.

[0005] Pyrometallurgical processes achieve the reduction and enrichment of metallic elements through high-temperature treatment, and the process is relatively simple and direct. However, its drawbacks are as follows: on the one hand, the process needs to be carried out at extremely high temperatures, resulting in huge energy consumption; on the other hand, this process has strict requirements for the pretreatment of raw materials. If impurities in the waste materials are not effectively removed, they are prone to forming compounds that affect the performance of the product at high temperatures, thereby adversely affecting the electrochemical performance of the recycled materials.

[0006] In contrast, direct regeneration technology, which can directly repair the material structure, shows great potential, but its core "lithium replenishment" step still faces challenges such as high energy consumption or complex processes.

[0007] Current mainstream direct regeneration technologies rely on high-temperature solid-state methods. This method typically involves mixing waste lithium iron phosphate materials with inorganic lithium sources (such as Li₂CO₃, LiOH·H₂O) and added carbon sources (such as sucrose), followed by long-term calcination at high temperatures (>650℃) under an inert atmosphere. However, this technical approach suffers from a series of inherent defects caused by the high-temperature process itself, specifically: ① High energy consumption and lithium loss: The high-temperature calcination process (usually lasting 6-14 hours) consumes a lot of energy and causes serious volatilization of active lithium elements (thermogravimetric-mass spectrometry analysis confirmed that the lithium loss rate is >15%), resulting in a lithium recovery rate of generally less than 85%, which leads to a secondary waste of resources.

[0008] ② Incomplete structural repair and impurity phase formation: High-temperature environments (>600℃) easily induce abnormal grain coarsening (SEM shows an average grain size increase of >50%), and exacerbate Fe³⁺ formation. + The lithium ion transport pathway transforms into a more thermodynamically stable, non-electrochemically active phase (such as Fe2O3) (characteristic peaks appear in XRD at 2θ≈33.2°). Simultaneously, high temperatures are insufficient to effectively heal microcracks within the particles caused by cycling, resulting in incomplete restoration of the lithium ion transport pathway.

[0009] ③ Poor carbon coating quality: The carbon layer formed by the pyrolysis of an external carbon source at high temperature has poor uniformity of distribution (SEM shows carbon element agglomeration) and weak interfacial bonding with the LiFePO4 matrix (peel strength <1.5 N / cm). During battery cycling, this carbon layer is prone to peeling, leading to instability in the electrode's electronic conductivity network, which manifests as accelerated capacity decay over long cycles (capacity retention is often below 80% after 300 cycles at 1C).

[0010] ④ Poor process compatibility and the influence of impurities: The high-temperature process is extremely sensitive to residual electrolytes, binders, and trace metal impurities (such as Al and Cu) that are difficult to completely remove from the precursor. These impurities easily participate in reactions at high temperatures, generating electrochemically inert or insulating impurity phases (such as LiAlO2, XRD 2θ≈18.5°), which not only reduces the proportion of active material but also hinders lithium-ion transport, seriously affecting the consistency between batches of recycled materials and the final electrochemical performance.

[0011] Therefore, developing a novel direct regeneration technology that can achieve deep structural repair, precise lithium compensation, and the construction of a stable conductive network at relatively low temperatures is key to breaking through the current industrialization bottleneck. Summary of the Invention

[0012] Therefore, the purpose of this invention is to propose a mild, simple, and highly effective method for the direct regeneration of waste lithium iron phosphate. The core of this method lies in utilizing a hydrothermal environment and leveraging the synergistic effect of lithium salts and specific organic solvents to achieve efficient lithium ion replenishment, in-situ repair of material defects, and functional coating of particle surfaces, thereby obtaining high-performance recycled materials in a one-step process.

[0013] In a first aspect, the present invention provides a low-temperature hydrothermal regeneration method for waste lithium iron phosphate, comprising the following steps: Step S1: Mix waste lithium iron phosphate powder with lithium salt, add it to the lining of a hydrothermal reactor, and mix it evenly with a magnetic stirrer to obtain a solid mixture; Step S2: Add a certain volume of oleylamine to the solid mixture of step S1, and continue to stir thoroughly in the liner of the reactor to form a uniform slurry-like reaction precursor; wherein, the mass-volume ratio of the waste lithium iron phosphate powder, lithium salt and oleylamine is 420~460 mg: 40~75 mg: 20~40 mL. Step S3: Seal the hydrothermal reactor and then place it in a constant temperature oven to carry out the hydrothermal reaction at the set temperature; after the reaction is completed, allow it to cool naturally to room temperature to obtain a suspension containing solid products; wherein, the hydrothermal reaction time is 10~20 h and the temperature is 100~200℃. Step S4: Transfer all the suspension obtained in step S3 to a centrifuge tube, centrifuge, discard the supernatant, and perform solid-liquid separation; Step S5: The solid product obtained in step S4 is repeatedly and alternately centrifuged and washed with anhydrous ethanol and deionized water to thoroughly remove residual organic matter and soluble impurities. The washed solid is placed in a vacuum drying oven for drying, and then ground to obtain gray-black regenerated lithium iron phosphate cathode material powder.

[0014] In the above scheme, oleylamine plays multiple key roles in this invention: First, as a dispersion medium and surfactant, it promotes the dispersion of inorganic lithium salts and the diffusion of lithium ions to defects in the bulk phase of the material; second, its molecules can undergo in-situ pyrolysis and carbonization on the surface of lithium iron phosphate particles during hydrothermal and subsequent processing, forming a uniform and dense conductive carbon coating layer.

[0015] A further embodiment is that the lithium salt is selected from lithium hydroxide and / or lithium acetate, preferably lithium hydroxide, and its function is to provide migratable lithium ions (Li) under hydrothermal conditions. + This is used to fill and repair lithium vacancies in the lattice of spent lithium iron phosphate. The lithium salt accounts for 5wt%-15wt% of the total mass.

[0016] A further embodiment is that the mass-to-volume ratio of the waste lithium iron phosphate powder, lithium salt, and oleylamine in the liquid is 450~460 mg: 40~50 mg: 20~40 mL.

[0017] A further option is that, in step S2, the stirring speed is 300~800 rpm and the time is 5~24 h.

[0018] A further proposed approach is to have the hydrothermal reaction in step S3 last for 10-14 hours at a temperature of 120-140°C.

[0019] A further approach is to include heating at a rate of 1-5°C / min in the hydrothermal reaction.

[0020] A further embodiment is that, in step S4, the centrifugation speed is 5000~12000 rpm and the time is 3~10 min; In step S5, the cleaning includes adding anhydrous ethanol and deionized water to the solid product and centrifuging at a speed of 5000~12000 rpm for 3~10 min, for a total of 2~10 centrifugations. In step S5, the drying temperature is 50~80℃ and the time is 6~18 h.

[0021] Secondly, the present invention provides a regenerated lithium iron phosphate obtained by the low-temperature hydrothermal regeneration method described above.

[0022] Thirdly, the present invention also provides the application of the above-described recycled lithium iron phosphate as a cathode material in lithium-ion batteries.

[0023] Fourthly, the present invention also provides a lithium-ion battery comprising the above-mentioned recycled lithium iron phosphate as the positive electrode material.

[0024] In the repair and recycling method provided by this invention, such as Figure 1 The diagram shows the principle of synergistic regeneration of waste lithium iron phosphate cathode material using lithium salt and oleylamine. Waste lithium iron phosphate powder and lithium salt are thoroughly mixed in an organic solvent to form a homogeneous precursor. Under mild hydrothermal reaction conditions, lithium ions (Li+) provided by the lithium salt are... +With the aid of organic solvents, lithium can be efficiently diffused into the bulk phase of the material and precisely embedded into lattice vacancies generated by cycling losses, achieving direct and mild lithium compensation. Simultaneously, the organic solvent (such as oleylamine) plays a multifunctional role in this system: on the one hand, it acts as a reaction medium to promote mass transfer and ion migration; on the other hand, its molecules undergo in-situ pyrolysis and carbonization under thermal action, forming a uniform, dense, and firmly bonded conductive carbon coating layer on the surface of the repaired lithium iron phosphate particles. This coating layer not only significantly improves the electronic conductivity of the material but also effectively suppresses side reactions and structural degradation during cycling. After the reaction, the material is alternately washed with deionized water and anhydrous ethanol to thoroughly remove excess lithium salts and organic residues adsorbed on the surface. Following low-temperature vacuum drying, a regenerated cathode material with a complete crystal structure and a clean surface is obtained. This method avoids high-temperature processes and simultaneously achieves structural repair and performance improvement under relatively mild conditions.

[0025] Compared with the prior art, the beneficial effects of the present invention are as follows: The core of this invention lies in achieving integrated regeneration of waste lithium iron phosphate materials through molecular design and process optimization under mild hydrothermal conditions, encompassing "lithium replenishment-structural repair-in-situ coating." This results in efficient and low-energy recycling of waste lithium iron phosphate materials. Compared to existing regeneration technologies, the advancements of this invention are mainly reflected in the following aspects: (1) The hydrothermal method is used to directly regenerate waste lithium iron phosphate cathode materials, which significantly reduces the reaction temperature. The entire process has the outstanding advantages of low energy consumption, short process, and environmental friendliness. The regenerated lithium iron phosphate material has a complete olivine crystal structure comparable to commercial materials and exhibits electrochemical characteristics such as high specific capacity, excellent rate performance and good cycle stability.

[0026] (2) The “lithium salt and specific organic solvent” synergistic system adopted in this invention can simultaneously achieve efficient replenishment of lithium ions, in-situ repair of material bulk defects, and in-situ construction of a uniform conductive carbon layer on the particle surface during hydrothermal process, thereby systematically repairing a variety of key defects in the material and ensuring the full recovery of the performance of the recycled material.

[0027] (3) The process conditions of this invention are mild, the requirements for raw material pretreatment are relatively relaxed, the process flow is simple and stable, and the equipment requirements are conventional. It has significant scalability and economic advantages, and is very suitable for large-scale industrial application. It provides a practical solution for the green and high-value recycling of waste lithium iron phosphate batteries.

[0028] (4) The lithium iron phosphate cathode material regenerated using the method of this invention has electrochemical performance that meets commercial requirements and can be directly used to manufacture lithium-ion battery cathodes and various types of lithium batteries. The assembled battery products have high specific capacity, excellent rate performance and long cycle life, and have broad application prospects in electric vehicles, energy storage systems and other fields. Attached Figure Description

[0029] Figure 1 A schematic diagram of the principle for regenerating waste lithium iron phosphate cathode materials using a hydrothermal method synergistically combining lithium salts and oleylamine; Figure 2 XRD patterns of waste lithium iron phosphate and recycled lithium iron phosphate cathode materials; Figure 3 XPS spectra of waste lithium iron phosphate and recycled lithium iron phosphate cathode materials; Figure 4 Long-cycle diagrams and charge-discharge curves of waste lithium iron phosphate and recycled lithium iron phosphate cathode materials at a current density of 0.2 C; Figure 5 The graph shows the electrochemical performance of waste lithium iron phosphate and recycled lithium iron phosphate cathode materials at different current densities. Detailed Implementation

[0030] To enable those skilled in the art to fully understand the technical solution and beneficial effects of the present invention, further detailed descriptions are provided below in conjunction with specific embodiments. It should be emphasized that this application is not limited to the following embodiments. The embodiments described are merely examples, and any embodiments with substantially the same structure and achieving the same effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that those skilled in the art can conceive of to the embodiments, and other ways of constructing embodiments by combining some of the constituent elements, are also included within the scope of this application without departing from the spirit of this application.

[0031] Unless otherwise specified, the experimental methods used in the embodiments of this invention are all conventional methods.

[0032] Unless otherwise specified, all materials and reagents used in the embodiments of this invention are commercially available.

[0033] The waste lithium iron phosphate powder used in this embodiment of the invention comes from Wuhan Ruikemei New Energy Co., Ltd.

[0034] Example 1 The organic solvent used in this embodiment is oleylamine, and the lithium salt used is lithium hydroxide.

[0035] I. Regenerated Lithium Iron Phosphate Materials The specific methods for regenerating waste lithium iron phosphate using these raw materials are as follows: 1) Add 453.63 mg of waste lithium iron phosphate (LiFePO4) powder and 46.37 mg of lithium hydroxide (LiOH) solid powder to the polytetrafluoroethylene liner of a 50 mL hydrothermal reactor and stir until homogeneous.

[0036] 2) Add 30 mL of oleylamine to the mixed powder from step 1) above, and stir in the reactor liner at a speed of 300 rad / s for 12 hours to form a uniform slurry precursor.

[0037] 3) After sealing the hydrothermal reactor, place it in a forced-air drying oven and heat it to 120°C at a heating rate of 2°C / min. Maintain this temperature for 12 hours. After the reaction is complete, allow it to cool naturally to room temperature to obtain a suspension containing the regenerated product.

[0038] 4) Transfer all the suspension obtained in step 3) to a centrifuge and centrifuge at 8000 rpm for 5 minutes, discarding the supernatant. Then, add anhydrous ethanol and deionized water to the solid in sequence, washing alternately under the same centrifugation conditions, repeating this washing process a total of 7 times.

[0039] 5) Transfer the cleaned solid product to a vacuum drying oven (P<10Pa) and dry at 70℃ for 12 hours. After drying, grind to obtain black powdered regenerated lithium iron phosphate cathode material (i.e., R-LFP in the attached diagram).

[0040] The regenerated lithium iron phosphate obtained in Example 1 was characterized by X-ray diffraction (XRD) and XPS, and the results are as follows: Figure 2-3 As shown.

[0041] Figure 2 The XRD patterns of waste lithium iron phosphate and bifunctional organolithium salts for regenerated lithium iron phosphate are shown in the figure. Compared with the standard card for lithium iron phosphate, waste lithium iron phosphate contains impurity peaks located at 18℃, 25℃, and 31℃. For the regenerated lithium iron phosphate, it is a perfect match with the standard card (PDF#83-2092). The main peaks of lithium iron phosphate are located at 17℃, 20℃, 25℃, 29℃, and 35℃, which is a typical olivine structure, proving that the waste lithium iron phosphate has been repaired.

[0042] Figure 3 The XPS spectra of waste lithium iron phosphate and recycled lithium iron phosphate are shown. For waste lithium iron phosphate, the peaks at 711.2 eV and 724.7 eV represent trivalent iron in Fe 2p3 / 2 and Fe 2p1 / 2, respectively, indicating that there is still a large amount of trivalent iron in waste lithium iron phosphate, mainly due to long-term battery cycling. For recycled lithium iron phosphate, the peaks at 709.9 eV and 723.3 eV represent divalent iron in Fe 2p3 / 2 and Fe 2p1 / 2, respectively. The absence of trivalent iron proves that the trivalent iron in waste lithium iron phosphate has been reduced, reducing the lithium iron phosphate antisite sites, thus yielding recycled lithium iron phosphate.

[0043] II. Half-cell applications The bifunctional organic lithium salt regenerated lithium iron phosphate cathode material obtained in Example 1 was used as an active material to prepare the cathode of a lithium-ion battery. The specific preparation method is as follows: the active material, acetylene black, and polyvinylidene fluoride are used as binders, mixed evenly and coated on aluminum foil, and dried to obtain a cathode sheet. The mass ratio of active material, acetylene black and polyvinylidene fluoride is 8:1:1.

[0044] Following the same method described above, a positive electrode sheet was made from the waste lithium iron phosphate in Example 1 as a control. Two different positive electrode plates were assembled into half-cells with pure lithium electrode plates, using LB-002 as the electrolyte. Electrochemical performance was then tested in the range of 2.5-4.0V. The results are shown below. Figure 4-5 As shown.

[0045] Figure 4 The electrochemical performance at 0.2 C of waste lithium iron phosphate (S-LFP) and regenerated lithium iron phosphate (R-LFP) was compared. Figure 4 The charge / discharge cycle of a includes cycles 1, 2, 5, and 10. The curves almost overlap, and the discharge plateau stabilizes at approximately 3.35 volts, indicating that the R-LFP cathode has a highly stable reaction potential and low polarization under 0.2C operating conditions. Its clear and stable plateau reflects good redox reversibility, proving that lithium ions can be efficiently inserted and extracted. Figure 4 Long-cycle performance tests of b show that at 0.2C (1C = 170 mA g) - At the current density of ¹), the initial discharge capacity of spent lithium iron phosphate (S-LFP) is only 63.8 mAh g⁻¹. In stark contrast, the capacity of recycled lithium iron phosphate (R-LFP) is significantly improved to 155.6 mAh g⁻¹. Notably, R-LFP achieves an excellent initial coulombic efficiency (ICE) of 100%, a significant improvement over the 90.5% of S-LFP, and also exhibits superior cycle stability.

[0046] Figure 5The rate electrochemical performance of waste lithium iron phosphate (S-LFP) and recycled lithium iron phosphate (R-LFP) was compared. Performance tests show that the discharge specific capacity of regenerated LFP (0.2C: 153.2 mAh / g, 0.5C: 47.9 mAh / g, 1C: 137.4 mAh / g, 2C: 123.5 mAh / g, 5C: 100.9 mAh / g) is significantly better than that of waste LFP (0.2C: 72.4 mAh / g, 0.5C: 61.6 mAh / g, 1C: 52.9 mAh / g, 2C: 43.0 mAh / g, 5C: 28.4 mAh / g). Furthermore, when the rate is switched back from 5C to 0.2C, the capacity of regenerated LFP can be restored to 153.3 mAh / g, indicating its excellent rate performance and meeting the standards for commercial LFP use.

[0047] In summary, the significant improvements in rate performance, reaction reversibility, and cycle stability of regenerated LFP demonstrate that this regeneration process effectively restores the lithium deficiency and ferric iron (Fe³⁺) content in waste LFP. + The high content of LFP and key structural defects such as Li / Fe reverse sites endow the regenerated LFP with excellent comprehensive electrochemical performance.

[0048] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that oleylamine in step 2) is replaced with an equal volume of anhydrous ethanol. The remaining steps are the same as in Example 1.

[0049] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that 46.37 mg of lithium hydroxide in step 1) was replaced with an equimolar amount of lithium carbonate. The remaining steps are the same as in Example 1.

[0050] Application examples The recycled lithium iron phosphate obtained in Example 1 and the comparative example were used as active materials to prepare the positive electrode of lithium-ion batteries. The specific method was as follows: the active material, acetylene black and polyvinylidene fluoride were mixed evenly in a mass ratio of 8:1:1 and then coated on aluminum foil. After drying, the positive electrode sheet was obtained. Following the same method, a positive electrode sheet was made using the waste lithium iron phosphate from Example 1 as a control; Two types of positive electrode sheets were assembled with pure lithium electrode sheets to form half-cells, with LB-002 as the electrolyte. Then, the electrochemical performance was tested in the range of 2.5-4.0V. The results are shown in Table 1 below.

[0051] Table 1. Electrochemical performance of half-cells assembled with different recycled lithium iron phosphate materials as cathodes

[0052] As shown in Table 1, the method for synergistic regeneration of waste lithium iron phosphate cathode materials using lithium hydroxide and oleylamine under low-temperature hydrothermal conditions provided by this invention significantly improves the rate performance, reaction reversibility, and cycle stability of the regenerated lithium iron phosphate material. This demonstrates that the bifunctional organic lithium salt and corresponding lithium iron phosphate regeneration process provided by this invention effectively repair the lithium deficiency and ferric iron (Fe³⁺) in waste lithium iron phosphate cathode materials. + Key structural defects such as excessively high Li / Fe content and too many Li / Fe reverse sites give regenerated lithium iron phosphate materials excellent comprehensive electrochemical performance.

[0053] Example 2 The difference between Example 2 and Example 1 is that in step 1), 40 mg of lithium hydroxide powder is mixed with 460 mg of waste lithium iron phosphate powder. The remaining steps are the same as in Example 1.

[0054] Example 3 The difference between Example 3 and Example 1 is that in step 1), 75 mg of lithium hydroxide powder is mixed with 425 mg of waste lithium iron phosphate powder. The remaining steps are the same as in Example 1.

[0055] Example 4 The difference between Example 4 and Example 1 is that the hydrothermal reaction temperature in step 3) is increased to 140°C. The remaining steps are the same as in Example 1.

[0056] Example 5 The difference between Example 5 and Example 1 is that the hydrothermal reaction temperature in step 3) is increased to 160°C. The remaining steps are the same as in Example 1.

[0057] Example 6 The difference between Example 6 and Example 1 is that the hydrothermal reaction time in step 3) is extended to 10 hours. The remaining steps are the same as in Example 1.

[0058] Example 7 The difference between Example 7 and Example 1 is that the hydrothermal reaction time in step 3) is extended to 14 hours. The remaining steps are the same as in Example 1.

[0059] Based on the preparation method of the positive electrode sheet in the application examples, the regenerated lithium iron phosphate obtained in Examples 2-7 was used as the active material to prepare the positive electrode of the lithium-ion battery.

[0060] The above positive electrode plates were assembled into a half cell, with LB-002 as the electrolyte. Then, the electrochemical performance was tested in the range of 2.5-4.0V. The results are shown in Table 2 below.

[0061] Table 2 Electrochemical performance of half-cells assembled with different recycled lithium iron phosphate materials as cathodes

[0062] As shown in Table 2, the present invention uses a hydrothermal method to directly regenerate waste lithium iron phosphate cathode materials, achieving integrated regeneration of "lithium replenishment-structural repair-in-situ coating" under mild hydrothermal conditions. The regenerated lithium iron phosphate materials all have good electrochemical properties.

[0063] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A low-temperature hydrothermal regeneration method for waste lithium iron phosphate, characterized in that, Includes the following steps: Step S1: Mix waste lithium iron phosphate powder with lithium salt to obtain a solid mixture; Step S2: Add oleylamine to the solid mixture and stir to obtain a slurry precursor; wherein the mass-to-volume ratio of the waste lithium iron phosphate powder, lithium salt and oleylamine is 420~460 mg: 40~75 mg: 20~40 mL. Step S3: The slurry precursor is subjected to a hydrothermal reaction and cooled to obtain a black suspension; wherein the hydrothermal reaction time is 10~20 h and the temperature is 100~200℃. Step S4: Centrifuge the black suspension, discard the supernatant, and obtain the solid product; Step S5: Clean and dry the solid product to obtain regenerated lithium iron phosphate.

2. The low-temperature hydrothermal regeneration method for waste lithium iron phosphate according to claim 1, characterized in that, In step S1, the lithium salt is selected from lithium hydroxide and / or lithium acetate, and the mass percentage of the lithium salt is 5wt% to 15wt%.

3. The low-temperature hydrothermal regeneration method for waste lithium iron phosphate according to claim 1, characterized in that, The mass-to-volume ratio of the waste lithium iron phosphate powder, lithium salt, and oleylamine in the liquid solution is 450~460 mg: 40~50 mg: 20~40 mL.

4. The low-temperature hydrothermal regeneration method for waste lithium iron phosphate according to claim 1, characterized in that, In step S2, the stirring speed is 300~800 rpm and the time is 5~24 h.

5. The low-temperature hydrothermal regeneration method for waste lithium iron phosphate according to claim 1, characterized in that, In step S3, the hydrothermal reaction takes 10-14 hours and the temperature is 120-140℃.

6. The low-temperature hydrothermal regeneration method for waste lithium iron phosphate according to claim 5, characterized in that, In hydrothermal reactions, heating is also carried out at a heating rate of 1~5℃ / min.

7. The low-temperature hydrothermal regeneration method for waste lithium iron phosphate according to claim 1, characterized in that, In step S4, the centrifugation speed is 5000~12000 rpm and the time is 3~10 min; In step S5, the cleaning includes adding anhydrous ethanol and deionized water to the solid product and centrifuging at a speed of 5000~12000 rpm for 3~10 min, for a total of 2~10 centrifugations. In step S5, the drying temperature is 50~80℃ and the time is 6~18 h.

8. A regenerated lithium iron phosphate obtained by the low-temperature hydrothermal regeneration method as described in any one of claims 1 to 7.

9. The application of regenerated lithium iron phosphate as described in claim 8 as a cathode material in lithium-ion batteries.

10. A lithium-ion battery, characterized in that, The recycled lithium iron phosphate as described in claim 8 is used as the cathode material.

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