A solid-phase regeneration method for waste lithium iron phosphate positive electrode material, regenerated lithium iron phosphate material and application thereof

Abstract

The application belongs to the technical field of battery material recycling and regeneration, and particularly relates to a solid-phase regeneration method of waste lithium iron phosphate positive electrode material, regenerated lithium iron phosphate material and application thereof. The application adopts a mixed organic lithium salt system composed of lithium citrate and lithium oxalate, realizes material repair by dissolving the mixed organic lithium salt system, mixing the solution with waste materials, drying, ball milling activation and multi-stage sintering treatment matching with lithium salt thermal decomposition characteristics in a protective atmosphere. The method utilizes the synergistic effect of the two organic lithium salts in decomposition temperature and chemical function, and simultaneously completes lithium supplement, deep reduction and in-situ conductive carbon coating in a single process. The regenerated lithium iron phosphate material has complete crystal structure, uniform surface coating, high specific capacity, excellent rate performance and cycle stability. The application is simple, efficient and environmentally friendly, and provides an effective solution for high-value direct regeneration of waste lithium iron phosphate.

Description

Technical Field

[0001] This invention belongs to the field of battery material recycling and regeneration technology, specifically relating to a solid-phase regeneration method for waste lithium iron phosphate cathode material, regenerated lithium iron phosphate material and its application. Background Technology

[0002] With the rapid development of new energy vehicles and the energy storage industry, lithium iron phosphate (LiFePO4) batteries have been widely used due to their high safety and long cycle life. Early-used batteries are gradually entering large-scale retirement, and a huge amount of waste is expected in the coming years. These waste batteries are rich in valuable elements such as lithium, iron, and phosphorus. If they cannot be efficiently recycled and reused, it will cause serious resource waste and environmental risks, and will also make it difficult to meet the market's continued demand for low-cost battery materials. Therefore, developing a recycling technology that can economically and environmentally restore the performance of waste lithium iron phosphate cathode materials is of vital importance for achieving closed-loop management of the entire battery lifecycle and sustainable industrial development.

[0003] Currently, the industry mainly focuses on several technical routes for the recycling and regeneration of waste lithium iron phosphate, including hydrometallurgy, pyrometallurgy, and direct regeneration and repair. Hydrometallurgy separates and purifies metal elements through acid leaching, extraction, and precipitation, achieving a high recovery rate. However, the process is lengthy and involves the consumption of large amounts of acid and alkali reagents and the generation of high-salt wastewater, while also completely destroying the valuable crystal structure of lithium iron phosphate. Pyrometallurgy achieves initial enrichment of metals through high-temperature smelting, but it consumes a huge amount of energy, lithium is easily lost through volatilization, and phosphorus recovery is also difficult. In contrast, direct regeneration and repair technology aims to restore the electrochemical performance of materials by replenishing lost lithium sources and repairing structural defects. Because of its relatively short process, lack of secondary pollution, and preservation of the original structural value of the material, it is considered a more promising development direction.

[0004] However, existing direct regeneration technologies still face significant bottlenecks in practical applications. Traditional solid-phase regeneration methods, which primarily use inorganic lithium sources such as lithium carbonate, struggle to achieve uniform dispersion at the molecular level through solid-solid mixing, easily leading to uneven composition of the regenerated material and localized lithium excess or deficiency. Furthermore, this method lacks effective in-situ reduction capabilities, making it difficult to completely repair Fe in the material. 3+ Oxidation defects often require the introduction of additional carbon sources or reducing agents, and prolonged high-temperature treatment is needed to promote lithium-ion diffusion. This can easily lead to excessive particle growth and agglomeration, impairing the rate performance of the material. Another hydrothermal remediation method, while improving homogeneity by utilizing a liquid-phase environment, requires the reaction to be carried out in an autoclave. This equipment is expensive, has high safety requirements, and is an intermittent operation, making it difficult to adapt to large-scale continuous industrial production scenarios. These factors collectively limit the application of existing technologies in high-performance, low-cost, and large-scale regeneration.

[0005] Therefore, the industry urgently needs to develop a new direct regeneration strategy that can fundamentally solve core problems such as poor uniformity of lithium source mixing, incomplete reduction and repair, high energy consumption or complex equipment, so as to truly realize the efficient, high-quality and environmentally friendly large-scale regeneration of waste lithium iron phosphate materials. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention proposes a solid-phase regeneration method for waste lithium iron phosphate cathode materials, the regenerated lithium iron phosphate materials, and their applications. This invention provides an innovative solution for obtaining high-performance regenerated lithium iron phosphate materials by employing a mixed organic lithium salt system of lithium citrate and lithium oxalate, combined with a specific multi-stage high-temperature sintering process.

[0007] The first objective of this invention is achieved through the following technical solution: A solid-phase regeneration method for waste lithium iron phosphate cathode material includes the following steps: S1. Lithium citrate and lithium oxalate are mixed and dissolved in a solvent to prepare a lithium replenishment solution, which is then mixed with waste lithium iron phosphate powder to obtain a slurry. S2. Dry the slurry obtained in step S1 to obtain precursor powder; S3. The precursor powder obtained in step S2 is subjected to ball milling activation treatment to obtain activated powder. S4. The powder activated in step S3 is subjected to multi-stage sintering under a protective atmosphere; the multi-stage sintering includes: First sintering stage: Heat to 300-400℃ and hold at that temperature; Second sintering stage: Heat to 500-580℃ and hold at that temperature; Third sintering stage: Heat to 680-750℃ and hold at that temperature; After multi-stage sintering, recycled lithium iron phosphate material is obtained.

[0008] Preferably, in step S1, the molar ratio of lithium citrate to lithium oxalate is 1:(0.8-1.5).

[0009] More preferably, the molar ratio of lithium citrate to lithium oxalate is 1:(1.0-1.2).

[0010] Preferably, in step S1, the waste lithium iron phosphate powder is a positive electrode active material powder obtained from retired batteries through dismantling, separation, and crushing.

[0011] Preferably, in step S1, the molar ratio of lithium to iron in the slurry is (1.02-1.15):1.

[0012] Preferably, in step S1, the solvent is a polar solvent.

[0013] More preferably, the polar solvent includes, but is not limited to, deionized water, ethanol, or acetone.

[0014] Preferably, in step S1, the slurry also includes other lithium salts.

[0015] More preferably, the lithium salt includes one or more of lithium acetate, lithium citrate, or lithium oxalate.

[0016] Preferably, in step S2, the drying is spray drying or vacuum drying.

[0017] More preferably, the inlet air temperature of the spray dryer is 180-250℃ and the outlet air temperature is 75-110℃.

[0018] More preferably, the vacuum drying temperature is 60-100℃ and the time is 4-12h.

[0019] Preferably, in step S3, the ball mill rotation speed is 300-800 rpm, the ball milling time is 1-6 h, and the ball-to-material mass ratio is (5-15):1.

[0020] Preferably, in step S4, the protective atmosphere is any one or more of nitrogen, argon, helium, and neon.

[0021] Preferably, in step S4, the heating rate of the first sintering stage is 2-5℃ / min, and the holding time is 2-4h.

[0022] Preferably, in step S4, the heating rate of the second sintering stage is 1-3℃ / min, and the holding time is 2-5h.

[0023] Preferably, in step S4, the heating rate of the third sintering stage is 2-5℃ / min, and the holding time is 6-10h.

[0024] The core of this solution lies in the use of a mixed organic lithium salt system composed of lithium citrate and lithium oxalate in a specific molar ratio. This system, combined with spray drying or vacuum drying, high-energy ball milling activation, and multi-stage sintering processes, achieves efficient and uniform regeneration of waste lithium iron phosphate materials through a unique molecular structure synergy and thermal decomposition sequence synergy mechanism. Specifically, lithium citrate undergoes thermal decomposition first at a low temperature (300-400℃), with its products forming a Li₂CO₃-Li₂O eutectic system in situ. This eutectic system can create a microscopic liquid phase environment at a lower temperature, significantly reducing the diffusion barrier of lithium ions in the solid phase, thereby achieving sufficient wetting of the waste particle surface and constructing a channel for rapid bulk diffusion of lithium ions. Subsequently, in the critical intermediate temperature stage (500-580℃), lithium oxalate, which has higher thermal stability, undergoes concentrated decomposition. This process simultaneously releases a large amount of CO reducing gas and produces a significant exothermic effect. The released high-concentration CO reducing atmosphere, aided by the previously established liquid phase channel, can effectively penetrate into the particle interior, removing the difficult-to-reduce Fe in the crystal lattice. 3+ (e.g., FePO4 impurity phase) is deeply reduced to Fe 2+ Simultaneously, the active lithium source released during decomposition, driven by the heat of reaction, is rapidly embedded into the vacated lattice sites after reduction through liquid-phase channels, precisely replenishing lithium vacancies and repairing lattice defects. Finally, the crystal undergoes final growth and healing at a high temperature (680-750℃). At the same time, the carbon skeleton remaining from the decomposition of lithium citrate undergoes graphitization at this temperature, forming a continuous and dense conductive carbon coating layer in situ, thereby simultaneously improving the structural integrity and surface conductivity of the crystal. This mixed lithium salt's relay-style decomposition characteristic of "liquid-phase diffusion followed by strong gas-phase reduction," precisely matched with a multi-stage sintering temperature window, synergistically solves the fundamental problems of uneven lithium source distribution, incomplete reduction, and slow solid-solid reaction kinetics in traditional solid-state methods.

[0025] The second objective of this invention is achieved through the following technical solution: A recycled lithium iron phosphate material, which is prepared by the method described in any of the above schemes.

[0026] Preferably, the Li / Fe molar ratio of the recycled lithium iron phosphate material is 1.08:1.

[0027] Preferably, the recycled lithium iron phosphate material has an olivine-type crystal structure.

[0028] Preferably, the surface of the recycled lithium iron phosphate material particles is coated with a continuous graphitized carbon layer.

[0029] Preferably, the carbon content of the recycled lithium iron phosphate material is 1.5-3.5 wt%.

[0030] Preferably, the tap density of the recycled lithium iron phosphate material is 1.20-1.45 g / cm³. 3 .

[0031] Preferably, the recycled lithium iron phosphate material has a discharge specific capacity of ≥155 mAh / g at 0.1C rate and a discharge specific capacity of ≥120 mAh / g at 5C rate.

[0032] The third objective of this invention is achieved through the following technical solution: A battery cathode comprising the recycled lithium iron phosphate material described in the above-described scheme.

[0033] Preferably, the positive electrode of the battery includes a current collector and a positive electrode active material layer coated on the current collector, wherein the positive electrode active material layer comprises the above-mentioned regenerated lithium iron phosphate material.

[0034] The fourth objective of this invention is achieved through the following technical solution: A battery comprising the positive electrode described in the above-described scheme.

[0035] Preferably, the battery is prepared by the following steps: mixing the recycled lithium iron phosphate material with a conductive agent and a binder to form a positive electrode slurry, coating it on an aluminum foil, drying it, rolling it, and then assembling it with a negative electrode, a separator, and an electrolyte to form a battery.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. By employing a specific combination of lithium citrate and lithium oxalate as a repair agent, a multi-functional one-step synergistic process of lithium replenishment, deep reduction, and conductive carbon coating is achieved. This combination utilizes the carbon skeleton characteristics of lithium citrate and the strong reducing properties of lithium oxalate to spontaneously complete all repair steps during heat treatment, without the need for additional carbon sources or reducing agents. This fundamentally simplifies the formulation system and ensures the uniformity of component action.

[0037] 2. The lithium citrate and lithium oxalate used in this invention both have good water solubility, which enables the uniform molecular-scale dispersion of the lithium source and waste lithium iron phosphate powder in the liquid phase by preparing a lithium replenishment solution. This method completely overcomes the problem of uneven raw material contact caused by solid-solid mechanical mixing in traditional solid-phase regeneration processes, laying an irreplaceable foundation for a uniform and consistent repair reaction in subsequent heat treatment.

[0038] 3. Based on the differences in decomposition temperature and characteristics of the two organic lithium salts, this invention constructs a relay repair mechanism that orderly connects "liquid-phase diffusion" and "gas-phase strong reduction". Lithium citrate decomposes first to form a liquid-phase diffusion channel, and lithium oxalate decomposes subsequently to provide a strong reducing atmosphere and reaction heat, thereby achieving efficient deep repair of defects deep in the crystal lattice under relatively mild sintering conditions.

[0039] 4. The recycled material prepared by this invention forms a uniform and highly conductive carbon coating layer in situ. This carbon layer originates from the thermal decomposition of lithium citrate and is tightly bonded to the recycled particles, significantly improving the electronic conductivity of the material. At the same time, the optimized sintering process inhibits excessive particle growth, giving the material excellent ion and electron transport capabilities.

[0040] 5. The entire process of this invention is short and efficient, avoiding complex wet processes and high-pressure equipment, and generating no harmful waste liquid. While efficiently restoring the electrochemical properties of materials, it significantly reduces energy consumption and environmental burden, and has excellent prospects for industrial application. Attached Figure Description

[0041] Figure 1 This is a scanning electron microscope image of the regenerated lithium iron phosphate material prepared in Example 1; Figure 2 The X-ray diffraction (XRD) pattern of the regenerated lithium iron phosphate material prepared in Example 1; Figure 3 The rate performance diagram is shown for the recycled lithium iron phosphate material prepared in Example 1.

[0042] Figure 4 The image shows a scanning electron microscope (SEM) image of the regenerated lithium iron phosphate material prepared in Comparative Example 3. Detailed Implementation

[0043] The following will provide a clear and complete description of the concept, specific structure, and technical effects of the present invention in conjunction with the embodiments and accompanying drawings, so as to fully understand the purpose, solution, and effects of the present invention. It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of the present invention can be combined with each other.

[0044] The materials used in the embodiments and comparative examples of this invention are described below: Waste lithium iron phosphate cathode powder: Obtained from commercially retired lithium iron phosphate batteries through discharge, disassembly, aluminum foil peeling, crushing, and sieving. Its quality indicators are: Li content 3.8-4.2 wt%, Fe content 32-36 wt% (determined by ICP-OES, method according to GB / T 24533), the main phase is olivine-type LiFePO4 with a small amount of FePO3 impurity phase (determined by XRD), and the volume distribution particle size Dv50 is 10 μm (determined by laser diffraction particle size analyzer, according to GB / T 19077).

[0045] Lithium citrate: purity ≥99%, purchased from Aladdin Reagents.

[0046] Lithium oxalate: purity ≥99%, purchased from Aladdin Reagent.

[0047] Lithium carbonate: purity ≥99.5%, purchased from Aladdin Reagents.

[0048] Sucrose: analytical grade, purchased from Sinopharm Group.

[0049] Lithium acetate: analytical grade, purchased from Aladdin Reagents.

[0050] Deionized water: resistivity ≥18.2 MΩ·cm.

[0051] Nitrogen: High-purity nitrogen, with a purity of ≥99.999%, used as a protective atmosphere.

[0052] Explanation of key terms: Volume distribution particle size Dv50: In this specification, it refers to the median diameter of the cumulative volume distribution of particles obtained by laser diffraction particle size analyzer (refer to standard GB / T19077), that is, at this particle size, the volume of particles smaller than and larger than this particle size in the sample each accounts for 50%.

[0053] The present invention will be further illustrated by specific embodiments below, but the protection of the present invention is not limited thereto.

[0054] Example 1 The preparation method of the recycled lithium iron phosphate material in Example 1 includes the following steps: (1) Weigh 100g of waste lithium iron phosphate powder. To make the Li / Fe molar ratio 1.08:1, calculate the total amount of lithium element to be added. Weigh 3.30g of lithium citrate and 1.73g of lithium oxalate according to the molar ratio of lithium citrate to lithium oxalate 1:1.08, and dissolve them together in 250mL of deionized water to prepare a lithium replenishment solution. Add the weighed 100g of waste lithium iron phosphate powder to the above solution and mechanically stir at 800 rpm for 45 minutes to obtain a slurry.

[0055] (2) The slurry obtained in step (1) is spray-dried. The inlet air temperature of the spray dryer is 200°C and the outlet air temperature is 95°C to obtain precursor powder.

[0056] (3) The precursor powder obtained in step (2) was subjected to ball milling activation treatment. The ball milling speed was 500 rpm, the ball milling time was 4 h, and the mass ratio of zirconia milling balls to material was 10:1 to obtain activated powder.

[0057] (4) The powder obtained in step (3) is subjected to multi-stage sintering under a nitrogen protective atmosphere. The multi-stage sintering is specifically as follows: the temperature is increased from room temperature to 350°C at a rate of 3°C / min and held for 3 hours; then the temperature is increased to 550°C at a rate of 1.5°C / min and held for 3 hours; finally, the temperature is increased to 720°C at a rate of 5°C / min and held for 8 hours. After sintering, the sintered product is crushed and passed through a 300-mesh sieve to obtain the recycled lithium iron phosphate material, which is the recycled lithium iron phosphate material of Example 1.

[0058] The morphology, structure, and electrochemical performance of the recycled lithium iron phosphate material prepared in Example 1 were characterized, and the results are as follows: The particle morphology was observed using a scanning electron microscope (SEM, Hitachi S-4800) to obtain... Figure 1 The SEM image shown indicates that the recycled material particles have regular morphology and uniform surface coating.

[0059] The phase composition was analyzed using an X-ray diffractometer (XRD, Bruker D8 Advance), and the results were obtained. Figure 2 The XRD pattern shown indicates that all diffraction peaks are consistent with standard olivine-type LiFePO4, and there are no impurities.

[0060] Example 2 The preparation method of the recycled lithium iron phosphate material in Example 2 includes the following steps: (1) Weigh 100g of waste lithium iron phosphate powder. To make the Li / Fe molar ratio 1.08:1, calculate the total amount of lithium element to be added. Weigh 2.84g of lithium citrate and 2.07g of lithium oxalate according to the molar ratio of lithium citrate to lithium oxalate 1:1.5, and dissolve them together in 250mL of deionized water to prepare a lithium replenishment solution. Add the weighed 100g of waste lithium iron phosphate powder to the above solution and mechanically stir at 800 rpm for 45 minutes to obtain a slurry.

[0061] (2-4) The regenerated lithium iron phosphate material of Example 2 was prepared according to step (2-4) of Example 1.

[0062] Example 3 The preparation method of the recycled lithium iron phosphate material in Example 3 includes the following steps: (1) Weigh 100g of waste lithium iron phosphate powder. To make the Li / Fe molar ratio 1.08:1, calculate the total amount of lithium element to be added. Weigh 3.13g of lithium citrate and 1.64g of lithium oxalate according to the molar ratio of lithium citrate to lithium oxalate 1:1.08, and add 0.267g of lithium acetate, which accounts for 5% of the total lithium molar amount. Dissolve all three in 250mL of deionized water to prepare a lithium replenishment solution. Add 100g of waste lithium iron phosphate powder to the above solution and mechanically stir at 800 rpm for 45 minutes to obtain a slurry.

[0063] (2-3) Proceed according to step (2-3) of Example 1.

[0064] (4) The powder obtained in step (3) was subjected to multi-stage sintering under a nitrogen protective atmosphere. The multi-stage sintering specifically consisted of: heating from room temperature to 320°C at a rate of 3°C / min and holding for 3 hours; then heating to 550°C at a rate of 1.5°C / min and holding for 3 hours; and finally heating to 720°C at a rate of 5°C / min and holding for 8 hours. After sintering, the sintered product was pulverized and passed through a 300-mesh sieve to obtain the recycled lithium iron phosphate material of Example 3.

[0065] Comparative Example 1 The preparation method of the recycled lithium iron phosphate material in Comparative Example 1 includes the following steps: (1) Weigh 100g of the same waste lithium iron phosphate powder as in Example 1. To make the Li / Fe molar ratio 1.08:1, calculate the total amount of lithium element to be added. Based on this, weigh 2.99g of lithium carbonate as a lithium supplement and weigh 2.68g of sucrose, which is equivalent to the theoretical carbon residue of the organic lithium salt in Example 1, as an external carbon source. Dry mechanically mix the waste lithium iron phosphate powder, lithium carbonate, and sucrose for 1 hour.

[0066] (2) The mixture obtained in step (1) is activated by ball milling, and the ball milling parameters are the same as in step (3) of Example 1.

[0067] (3) The powder obtained in step (2) is sintered under a nitrogen protective atmosphere. The sintering process is the same as step (3) of Example 1 to obtain the recycled lithium iron phosphate material of Comparative Example 1.

[0068] Comparative Example 2 The preparation method of the recycled lithium iron phosphate material in Comparative Example 2 includes the following steps: (1) Weigh 100g of the same waste lithium iron phosphate powder as in Example 1. To make the Li / Fe molar ratio 1.08:1, calculate the total amount of lithium element to be added. Using only lithium citrate as the lithium salt, weigh 5.67g of lithium citrate and dissolve it in 250mL of deionized water to prepare a lithium replenishment solution. Add 100g of the weighed waste lithium iron phosphate powder to the above solution and mechanically stir at 800 rpm for 45 minutes to obtain a slurry.

[0069] (2-4) Following the steps (2-4) of Example 1, the recycled lithium iron phosphate material of Comparative Example 2 was obtained.

[0070] Comparative Example 3 The preparation method of the recycled lithium iron phosphate material in Comparative Example 3 includes the following steps: (1) Weigh 100g of the same waste lithium iron phosphate powder as in Example 1. To make the Li / Fe molar ratio 1.08:1, calculate the total amount of lithium element to be added. Using only lithium oxalate as the lithium salt, weigh 4.13g of lithium oxalate and dissolve it in 250mL of deionized water to prepare a lithium replenishment solution. Add 100g of the weighed waste lithium iron phosphate powder to the above solution and mechanically stir at 800 rpm for 45 minutes to obtain a slurry.

[0071] (2-4) The recycled lithium iron phosphate material of Comparative Example 3 was prepared according to step (2-4) of Example 1.

[0072] The particle morphology of the recycled lithium iron phosphate material in Comparative Example 3 was observed using a scanning electron microscope (SEM, Hitachi S-4800). Figure 4 The SEM image shown indicates that the particles have irregular morphology and discontinuous surface coating.

[0073] Comparative Example 4 The preparation method of the recycled lithium iron phosphate material in Comparative Example 4 includes the following steps: (1-3) Proceed according to step (1-3) of Example 1.

[0074] (4) The powder obtained in step (3) is subjected to multi-stage sintering under a nitrogen protective atmosphere. The multi-stage sintering is specifically as follows: the temperature is increased from room temperature to 550°C at a rate of 3°C / min and held for 3 hours; then the temperature is decreased to 350°C at a rate of 1.5°C / min and held for 3 hours; finally, the temperature is increased to 720°C at a rate of 5°C / min and held for 8 hours. After sintering, the sintered product is crushed and passed through a 300-mesh sieve to obtain the recycled lithium iron phosphate material of Comparative Example 4.

[0075] Comparative Example 5 The preparation method of the recycled lithium iron phosphate material in Comparative Example 5 includes the following steps: (1-3) Proceed according to step (1-3) of Example 1.

[0076] (4) The powder obtained in step (3) is subjected to multi-stage sintering under a nitrogen protective atmosphere. The multi-stage sintering is specifically as follows: the temperature is increased from room temperature to 350°C at a rate of 3°C / min and held for 3 hours; then the temperature is increased to 450°C at a rate of 1.5°C / min and held for 3 hours; finally, the temperature is increased to 720°C at a rate of 5°C / min and held for 8 hours. After sintering, the sintered product is crushed and passed through a 300-mesh sieve to obtain the recycled lithium iron phosphate material of Comparative Example 5.

[0077] Comparative Example 6 The preparation method of the recycled lithium iron phosphate material in Comparative Example 6 includes the following steps: (1) Weigh 100g of the same waste lithium iron phosphate powder as in Example 1. To make the Li / Fe molar ratio 1.08:1, calculate the total amount of lithium element to be added. Only lithium oxalate is used as the lithium salt, and 4.13g of lithium oxalate is weighed; and an additional 2.68g of sucrose, equivalent to the theoretical carbon residue of lithium citrate in Example 1, is weighed as a carbon source. Lithium oxalate and sucrose are dissolved / dispersed together in 250mL of deionized water to prepare a lithium replenishment solution. Add 100g of the weighed waste lithium iron phosphate powder to the above solution and mechanically stir at 800 rpm for 45 minutes to obtain a slurry.

[0078] (2-4) The recycled lithium iron phosphate material of Comparative Example 6 was prepared according to step (2-4) of Example 1.

[0079] Comparative Example 7 The preparation method of the recycled lithium iron phosphate material in Comparative Example 7 includes the following steps: (1) Weigh 100g of the same waste lithium iron phosphate powder as in Example 1. To make the Li / Fe molar ratio 1.08:1, calculate the total amount of lithium element to be added. Weigh 4.25g of lithium citrate and 1.03g of lithium oxalate according to the lithium citrate to lithium oxalate molar ratio of 1:0.5, and dissolve them together in 250mL of deionized water to prepare a lithium replenishment solution. Add 100g of the weighed waste lithium iron phosphate powder to the above solution and mechanically stir at 800 rpm for 45 minutes to obtain a slurry.

[0080] (2-4) Following the steps (2-4) of Example 1, the recycled lithium iron phosphate material of Comparative Example 7 was obtained.

[0081] Comparative Example 8 The preparation method of the recycled lithium iron phosphate material in Comparative Example 8 includes the following steps: (1) Weigh 100g of the same waste lithium iron phosphate powder as in Example 1. To make the Li / Fe molar ratio 1.08:1, calculate the total amount of lithium element to be added. Weigh 2.43g of lithium citrate and 2.36g of lithium oxalate according to the lithium citrate to lithium oxalate molar ratio of 1:2.0, and dissolve them together in 250mL of deionized water to prepare a lithium replenishment solution. Add 100g of the weighed waste lithium iron phosphate powder to the above solution and mechanically stir at 800 rpm for 45 minutes to obtain a slurry.

[0082] (2-4) Following the steps (2-4) of Example 1, the recycled lithium iron phosphate material of Comparative Example 8 was obtained.

[0083] Comparative Example 9 The preparation method of the recycled lithium iron phosphate material in Comparative Example 9 includes the following steps: (1) Prepare the slurry according to step (1) of Example 1 and spray dry it to obtain precursor powder.

[0084] (2) The ball milling activation step is omitted, and the precursor powder obtained in step (1) is sintered directly.

[0085] (3) The powder was sintered under a nitrogen protective atmosphere according to the multi-stage sintering process of step (4) in Example 1. After sintering, the sintered product was crushed and passed through a 300-mesh sieve to obtain the recycled lithium iron phosphate material of Comparative Example 9.

[0086] Comparative Example 10 The preparation method of the recycled lithium iron phosphate material of Comparative Example 10 includes the following steps: (1) Weigh 100g of the same waste lithium iron phosphate powder as in Example 1. To make the Li / Fe molar ratio 1.08:1, calculate the total amount of lithium element to be added. Using a combination of lithium acetate and lithium citrate, with the total lithium molar amount the same as in Example 1 and the molar ratio set at 1:1.08, weigh 4.34g of lithium citrate and 1.26g of lithium acetate, and dissolve them together in 250mL of deionized water to prepare a lithium replenishment solution. Add 100g of the weighed waste lithium iron phosphate powder to the above solution and mechanically stir at 800 rpm for 45 minutes to obtain a slurry.

[0087] (2-4) Following the steps (2-4) of Example 1, the recycled lithium iron phosphate material of Comparative Example 10 was obtained.

[0088] Comparative Example 11 The preparation method of the recycled lithium iron phosphate material of Comparative Example 11 includes the following steps: (1) Weigh 100g of the same waste lithium iron phosphate powder as in Example 1. To make the Li / Fe molar ratio 1.08:1, calculate the total amount of lithium element to be added. Using a combination of lithium oxalate and lithium carbonate at a ratio of 1:1.08, weigh 1.56g of lithium carbonate and 1.99g of lithium oxalate, and dissolve them together in 250mL of deionized water to prepare a lithium replenishment solution. Add 100g of the weighed waste lithium iron phosphate powder to the above lithium replenishment solution and mechanically stir at 800 rpm for 45 minutes to obtain a slurry.

[0089] (2-4) Following the steps (2-4) of Example 1, the recycled lithium iron phosphate material of Comparative Example 11 was obtained.

[0090] The recycled lithium iron phosphate materials prepared in Examples 1-3 and Comparative Examples 1-11 were assembled into coin cells according to the following steps for performance testing. The results are shown in Table 1.

[0091] (a) The preparation of the mold battery includes the following steps: (1) Electrode slurry preparation: Weigh 0.8 g of recycled lithium iron phosphate material, 0.1 g of conductive agent (Super P) and 0.1 g of binder (polyvinylidene fluoride, PVDF). First, dissolve PVDF in N-methylpyrrolidone (NMP) solvent to form a homogeneous slurry. Then add the active material and conductive agent, and stir in a planetary mixer. First, stir at a low speed of 500 rpm for 30 minutes to mix the raw materials, and then stir at a high speed of 2000 rpm for 60 minutes. The solid content of the slurry is controlled at 40%.

[0092] (2) Electrode coating and drying: The above slurry is uniformly coated on an aluminum foil current collector with a thickness of 20 μm, and the wet film thickness is controlled to be 150 μm using a coating machine. The coated electrode is then transferred to a vacuum drying oven and dried at 100°C for 12 hours to completely remove the solvent.

[0093] (3) Electrode rolling and cutting: The dried electrode is cut into round pieces with a diameter of 13 mm using a die.

[0094] (4) Button cell assembly: All battery assembly operations were carried out in an argon-filled glove box (water and oxygen content both below 0.1 ppm). A lithium metal sheet was used as the counter electrode and reference electrode, a Celgard 2400 polypropylene membrane as the separator, and a 1 mol / L LiPF6 EC / DMC (volume ratio 1:1) solution as the electrolyte. The cells were assembled sequentially in the following order: positive electrode shell, positive electrode sheet, separator (wetted with electrolyte), lithium sheet, gasket, spring sheet, and negative electrode shell. The cells were then sealed using a sealing machine to produce CR2032 type button cells. After assembly, the cells were allowed to stand for 24 hours before electrochemical testing.

[0095] (ii) Tap density test Test standard: 10 g of powder sample is placed in a calibrated graduated cylinder and vibrated with a tapped density meter (model JZ-1) at an amplitude of 3 mm and a frequency of 300 times / minute until the powder volume no longer changes. The final volume (V) is recorded. Tapped density is calculated as ρ = m / V.

[0096] (III) Electrochemical performance testing The test was conducted using the Wuhan Landian CT2001A battery testing system.

[0097] Charge-discharge test: The discharge specific capacity was tested at different rates within the voltage range of 2.5-4.2 V. First, activation was performed by three charge-discharge cycles at a rate of 0.1C (1C = 170 mA / g), and then the discharge performance at 0.1C, 1C, and 5C rates was tested respectively.

[0098] Cyclic performance test: Constant current charge-discharge cycle test was performed at 1C rate, and the capacity retention rate was recorded after 100 cycles.

[0099] Table 1. Performance test results of the recycled lithium iron phosphate materials prepared in Examples 1-3 and Comparative Examples 1-6 The test results show that the lithium iron phosphate material regenerated using the method of this invention exhibits significantly better overall electrochemical performance than traditional solid-state methods and single-component remediation routes. Example 1, serving as a performance benchmark, achieved a discharge specific capacity of 164.2 mAh / g at 0.1C, close to the theoretical value of the material. Simultaneously, it maintained a high capacity of 131.8 mAh / g at a high rate of 5C, and after 100 cycles, the capacity showed almost no decay, with a retention rate as high as 99.5%. This demonstrates the comprehensive advantages of this method in restoring capacity, increasing rate capability, and ensuring long cycle life. Example 2 adjusted the mixed lithium salt ratio to 1:1.5, and the regenerated material still maintained a similar high capacity and cycle stability as Example 1. This indicates that high-performance regenerated materials can be obtained by varying the ratio within the range described in the claims, proving the fault tolerance and versatility of the technical solution. Example 3 introduced a small amount of lithium acetate into the baseline formulation and fine-tuned the first-stage sintering temperature. Its material properties were basically the same as those in Example 1. This further demonstrates that the process framework of the present invention has a certain degree of flexibility and adjustability, and can accommodate small process adjustments without impairing the performance of the final product.

[0100] Comparing Example 1 and Comparative Example 1, it can be seen that when only lithium carbonate is used and sucrose is added as a carbon source, all performance indicators of the material decline across the board, especially the 5C rate capacity and cycle stability, which decrease significantly. This indicates that the simple combination of traditional inorganic lithium source and added carbon source is insufficient to achieve uniform repair and effective conductive network construction. Without adding a carbon source, the performance deteriorates further.

[0101] By comparing Example 1 and Comparative Example 2, it can be seen that when only lithium citrate is used in the repair system and lithium oxalate is missing, the medium-to-high rate performance and cycle stability of the material show an observable decrease. This indicates that the strong reducing atmosphere and concentrated exothermic reaction provided by the absence of lithium oxalate in the medium temperature stage are insufficient to support the repair of deep defects and rapid electrochemical reactions.

[0102] By comparing Example 1 and Comparative Example 3, it can be seen that when only lithium oxalate is used in the repair system and lithium citrate is missing, the tap density and electrochemical performance at all magnifications of the material deteriorate significantly. This confirms that the lack of low-temperature liquid phase formation and carbon skeleton function provided by lithium citrate leads to fragile particle structure and poor conductivity, which fundamentally limits the diffusion of lithium ions and the transport of electrons.

[0103] By comparing Example 1 and Comparative Example 4, it can be seen that when the multi-stage sintering sequence is reversed, and medium-temperature reduction is performed first and then low-temperature heat treatment is performed, the material's capacity and tap density both decrease. This highlights that the relay sequence of "first constructing liquid phase channels and then performing strong gas phase reduction" designed in this invention is indispensable for achieving orderly and efficient repair.

[0104] By comparing Example 1 and Comparative Example 5, it can be seen that when the sintering temperature in the second stage does not reach the optimal decomposition window of lithium oxalate, the high-rate performance of the material degrades. This indicates that the temperature setting that precisely matches the decomposition characteristics of lithium oxalate is the key to releasing its deep reduction potential.

[0105] By comparing Example 1 and Comparative Example 6, it can be seen that even with the same solution mixing and sintering process, the repair effect of the "patchwork" scheme of lithium oxalate with added sucrose is still far inferior to that of the in-situ synergistic system of lithium citrate and lithium oxalate. This further confirms the unique value of the two organic lithium salts in complementing and promoting each other's functions during thermal decomposition, rather than a simple superposition of functions.

[0106] By comparing Example 1 and Comparative Example 7, it can be seen that when the molar ratio of lithium citrate to lithium oxalate is too low (insufficient lithium oxalate ratio), the high-rate performance of the material decreases. This indicates that insufficient lithium oxalate ratio will weaken the reduction and repair effect in the intermediate temperature stage and make it difficult to fully exert the synergistic effect.

[0107] By comparing Example 1 and Comparative Example 8, it can be seen that when the molar ratio of lithium citrate to lithium oxalate is too high (the proportion of lithium oxalate is too high), the tap density and high-rate performance of the material both decrease. This indicates that excessive lithium oxalate will destroy the carbon network structure constructed by lithium citrate, resulting in poor particle density and reduced conductivity.

[0108] By comparing Example 1 and Comparative Example 9, it can be seen that when the ball milling activation step is omitted, the various properties of the material all decrease to varying degrees. This indicates that ball milling activation can effectively break up soft agglomerates in the precursor and promote uniform contact between lithium salt and waste particles, which is a necessary step to optimize material performance.

[0109] By comparing Example 1 and Comparative Example 10, it can be seen that when lithium acetate is used to replace the combination of lithium oxalate and lithium citrate, the high-rate performance and cycle stability of the material are not as good as those of Example 1. This indicates that although lithium acetate can provide a lithium source, its decomposition characteristics cannot replace the strong reduction function of lithium oxalate in the intermediate temperature stage, and it cannot achieve the same degree of deep defect repair.

[0110] By comparing Example 1 and Comparative Example 11, it can be seen that when lithium carbonate and lithium oxalate are combined, the overall performance of the material is significantly worse than that of Example 1. This indicates that lithium carbonate does not have the in-situ carbonization and liquid phase coating functions of lithium citrate, and cannot construct the conductive network and particle morphology required for high-performance recycled materials.

[0111] In summary, this invention has developed a highly efficient and synergistic regeneration technology through a specific combination of lithium citrate and lithium oxalate, solution-level uniform mixing, and multi-stage sintering precisely matched with thermal decomposition characteristics, which has successfully restored the superior electrochemical performance of waste lithium iron phosphate materials.

[0112] The specific embodiments described herein are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains may make various modifications or additions to the described specific embodiments or use similar methods to substitute them without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A solid-phase regeneration method for waste lithium iron phosphate cathode material, characterized in that, Includes the following steps: S1. Lithium citrate and lithium oxalate are mixed and dissolved in a solvent to prepare a lithium replenishment solution, which is then mixed with waste lithium iron phosphate powder to obtain a slurry. S2. Dry the slurry obtained in step S1 to obtain precursor powder; S3. The precursor powder obtained in step S2 is subjected to ball milling activation treatment to obtain activated powder. S4. The powder activated in step S3 is subjected to multi-stage sintering under a protective atmosphere; the multi-stage sintering includes: First sintering stage: Heat to 300-400℃ and hold at that temperature; Second sintering stage: Heat to 500-580℃ and hold at that temperature; Third sintering stage: Heat to 680-750℃ and hold at that temperature; After multi-stage sintering, recycled lithium iron phosphate material is obtained.

2. The method according to claim 1, characterized in that, In step S1, the molar ratio of lithium citrate to lithium oxalate is 1:(0.8-1.5).

3. The method according to claim 1, characterized in that, In step S2, the drying is either spray drying or vacuum drying.

4. The method according to claim 3, characterized in that: The inlet air temperature of the spray dryer is 180-250℃, and the outlet air temperature is 75-110℃. And / or, the vacuum drying temperature is 60-100℃ and the time is 4-12h.

5. The method according to claim 1, characterized in that, In step S3, the ball mill rotation speed is 300-800 rpm, the ball milling time is 1-6 h, and the ball-to-material mass ratio is (5-15):

1.

6. The method according to claim 1, characterized in that, In step S4, the heating rate of the first sintering stage is 2-5℃ / min, and the holding time is 2-4h; the heating rate of the second sintering stage is 1-3℃ / min, and the holding time is 2-5h; the heating rate of the third sintering stage is 2-5℃ / min, and the holding time is 6-10h.

7. The method according to claim 1, characterized in that: In step S1, the solvent is water, ethanol, or a mixture thereof; And / or, in step S1, the molar ratio of lithium to iron in the slurry is (1.02-1.15):1; And / or, in step S4, the protective atmosphere is any one or more of nitrogen, argon, helium, and neon.

8. A recycled lithium iron phosphate material, characterized in that, It is prepared by any one of claims 1 to 7.

9. A battery positive electrode, characterized in that, It comprises the recycled lithium iron phosphate material as described in claim 8.

10. A battery, characterized in that, It comprises the battery positive electrode as described in claim 9.

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