Method for realizing efficient regeneration of uniform carbon-coated lithium iron phosphate by hydrothermal lithium compensation-transient sintering

Abstract

The invention discloses a method for realizing efficient regeneration of uniform carbon-coated lithium iron phosphate through hydrothermal lithium compensation-transient sintering, and belongs to the technical field of new energy material recycling. Aiming at the defect that a conductive carbon layer cannot be repaired by an existing hydrothermal regeneration technology, the method comprises the following steps: firstly, carrying out lattice lithium supplement and structure repair on a waste lithium iron phosphate positive electrode material through a hydrothermal reaction, then mixing the repair material with a functional carbon source, and carrying out low-temperature transient sintering in an inert atmosphere, and a uniform and compact conductive carbon coating layer is formed on the surface of the material in situ, so that synergistic regeneration of a bulk phase and the surface is realized. The method is short in technological process, green and low-carbon, the electrochemical performance of the regenerated material is recovered to the commercial level, the first discharge capacity is larger than or equal to 155 mAh / g, the hundred-circle capacity retention ratio is larger than or equal to 97%, and the method is suitable for large-scale recycling of the decommissioned lithium iron phosphate batteries.

Description

Technical Field

[0001] This invention belongs to the field of new energy technology, specifically relating to a method and application for achieving efficient regeneration of lithium iron phosphate through uniform carbon coating of lithium-ion battery cathode materials. Background Technology

[0002] With advancements in lithium iron phosphate (LFP) nano-sizing strategies and powder gradation processes, its application in the electric vehicle sector has been increasing year by year, and it is trending towards surpassing the market share of ternary lithium batteries. It is estimated that global LFP cell production will increase by approximately 51% year-on-year in 2025, reaching a total output of around 1300 GWh. LFP batteries face the challenge of continuous loss of active lithium and the resulting lithium iron phosphate antisite defects during use, leading to a limited lifespan of approximately 5-8 years. The approaching retirement of power batteries has resulted in a surge in the number of used batteries on the market. Therefore, developing direct recycling methods suitable for power LFP batteries will contribute to the circular economy, reduce the over-exploitation of lithium metal resources, and save battery costs.

[0003] Currently, industrialized battery recycling methods include pyrometallurgical recycling and hydrometallurgical recycling. Pyrometallurgical recycling utilizes a lattice-breaking and metal-extraction technology to recover lithium metal. It employs high-temperature smelting to remove impurities such as graphite, binders, and separators, and breaks down the lattice of the LFP cathode material, forming mixed oxides or complex salts. Lithium metal is then extracted through acid leaching and precipitation. The core problems of pyrometallurgical recycling are low lithium recovery rates, lithium volatilization at high temperatures, and high energy consumption. Furthermore, the pyrolysis of binders generates highly toxic fluorine gases, and improper slag disposal can easily cause soil and groundwater pollution, posing significant drawbacks. Hydrometallurgical recycling, on the other hand, requires large amounts of high-concentration acids and strong oxidants to leach structurally stable LFP, resulting in high actual costs. This leads to the generation of large amounts of acidic and alkaline wastewater and wastewater rich in organic pollutants after leaching, resulting in high investment and operating costs for treatment facilities and significant environmental pressure. In addition, the hydrometallurgical process involves multiple steps such as pretreatment, leaching, impurity removal, extraction, precipitation, and drying, requiring multiple filtration and washing processes, resulting in long production cycles and low efficiency. In summary, both pyrometallurgical and hydrometallurgical recycling methods aim to extract a single metal, which wastes the unique olivine lattice structure advantage of LFP. Furthermore, due to the lithium content of LFP being only about 3.9 wt.% and the hydrophobic properties caused by the carbon coating, traditional methods not only struggle to achieve efficient separation and targeted recovery of lithium, but also suffer from common problems such as low extraction efficiency and high cost, making them unsuitable for the recycling needs of LFP batteries.

[0004] Compared to the inherent limitations of the pyrometallurgical route, which involves "destroying the crystal lattice and extracting metals," such as high energy consumption, low recovery rate, and environmental pressure, the industry urgently needs to develop recycling technologies that can preserve the original crystal lattice structure of lithium iron phosphate (LFP) while balancing efficiency and environmental protection. Against this backdrop, direct recycling processes centered on "remediation and regeneration" have emerged. Hydrothermal direct recovery technology, a revolutionary approach in LFP battery recycling, operates on the core logic of "preserving the original crystal structure and targeting defect repair." It completely abandons the extensive "destruction-extraction" of traditional pyrometallurgical methods and the "dissolution-separation" of wet methods. Compared to traditional recycling technologies, direct recovery offers significant advantages: high lithium recovery and resource utilization rates; through targeted lithium replenishment and crystal repair, it fully preserves the original stoichiometry of elements such as Fe and P in LFP, eliminating the need for additional iron or phosphorus sources; significantly reduced energy consumption and environmental impact, with reaction temperatures only 1 / 5 to 1 / 3 of pyrometallurgical methods, unit energy consumption less than 30% of pyrometallurgical methods, and no need for strong acids, oxidants, or chlorinators. Wastewater generation is reduced by more than 90% compared to wet methods, and exhaust emissions are nearly zero, resulting in an 80% reduction in environmental treatment costs. Therefore, it has received widespread attention from the scientific and industrial communities. However, during cycling, lithium iron phosphate cathode materials undergo repeated lithium-ion insertion and extraction, as well as electrolyte corrosion, leading to the destruction of the carbon layer coating on their surface and a decrease in conductivity. However, the hydrothermal lithium replenishment process focuses on repairing the material composition and cannot simultaneously repair the surface carbon layer. Therefore, in order to improve the electrochemical performance of recycled lithium iron phosphate, it is crucial to develop a direct recovery process involving hydrothermal lithium replenishment and sintering with carbon coating. Summary of the Invention

[0005] The primary objective of this invention is to overcome the shortcomings of existing technologies, particularly the simple hydrothermal regeneration method, and to provide a highly efficient and environmentally friendly method for regenerating lithium iron phosphate cathode materials. This method aims to target and repair lithium deficiency and lattice defects in the bulk phase of the material through a hydrothermal reaction, and subsequently, through a controllable low-temperature sintering process, form a uniform and dense conductive carbon coating layer in situ on the surface of the repaired material. This achieves dual regeneration of the material's structure and function, ensuring its electrochemical performance meets commercial requirements.

[0006] Another object of the present invention is to provide a high-performance regenerated lithium iron phosphate cathode material prepared by the above method and its application in lithium-ion batteries.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] This invention provides a method for achieving efficient regeneration of lithium iron phosphate through uniform carbon coating, comprising the following steps:

[0009] Pretreatment and separation: The retired lithium iron phosphate batteries are discharged and disassembled to obtain the positive electrode sheet. The electrode sheet is immersed in an organic solvent such as N-methylpyrrolidone (NMP) and heat-treated at 100-140°C for 8-15 hours to dissolve the binder (such as PVDF), thereby separating the degraded lithium iron phosphate positive electrode material from the aluminum foil current collector. After cleaning and drying, the degraded positive electrode material powder to be repaired is obtained.

[0010] Hydrothermal lithium replenishment repair: The degraded cathode material described above is mixed with an aqueous solution containing a lithium compound (such as lithium hydroxide) and an organic reducing agent (such as citric acid), and placed in a hydrothermal reactor. The reaction is carried out at 80–120°C, preferably 100–120°C, for 3–6 hours. This step utilizes the hydrothermal environment to promote the diffusion of lithium ions into the LFP lattice, while the reducing agent inhibits Fe... 3+ The generation of [something] repairs crystal structure defects caused by lithium iron antisites, etc., and restores the correct stoichiometry and crystal integrity of the material.

[0011] Carbon-coated sintering regeneration: The material after hydrothermal lithium replenishment is washed and dried, then uniformly mixed with a functional carbon source (such as sucrose, glucose, dopamine, etc.), wherein the amount of carbon source added is 2-6 wt% of the mass of the degraded cathode material, preferably 4-5 wt%. The mixing method is preferably ball milling, with a milling time of 2-6 hours, preferably 4-6 hours. The uniformly mixed material is then briefly sintered under an inert atmosphere (such as nitrogen or argon) at a sintering temperature of 600-800℃, preferably 650-750℃, most preferably 700℃, for 8-12 hours. During this process, the carbon source pyrolyzes and forms a continuous, uniform, and highly conductive amorphous carbon coating layer on the surface of the lithium iron phosphate particles.

[0012] By organically combining the two key steps of "hydrothermal lithium replenishment" and "brief sintering", this invention not only replenishes active lithium and repairs the crystal lattice in bulk, but also reconstructs efficient electronic conductivity pathways on the surface, thereby achieving comprehensive regeneration of degraded lithium iron phosphate materials.

[0013] Compared with the prior art, the present invention has the following significant advantages:

[0014] 1. The regenerated lithium iron phosphate material of this invention has an initial discharge capacity of over 155 mAh / g and a capacity retention rate of over 97% after 100 cycles. Its comprehensive electrochemical performance is comparable to that of commercial new materials and can be directly used to assemble new batteries.

[0015] 2. This invention eliminates the use of strong acids and strong oxidants, resulting in minimal wastewater and waste gas emissions; the reaction temperature is much lower than that of pyrometallurgical processes, reducing energy consumption by more than 60%; the process flow is simple, requiring no complex separation and purification steps, achieving efficient and low-carbon recovery.

[0016] 3. This invention fully utilizes and repairs the original olivine crystal structure of LFP, eliminating the need for additional iron or phosphorus sources and maximizing the value of the material itself.

[0017] 4. The invention creatively solves the industry technical problem that the single hydrothermal method cannot repair the conductive carbon layer. Through the synergistic process of "repairing the bulk phase first and then coating the conductive layer", it achieves a regeneration effect of 1+1>2, providing a practical and feasible industrialization technical solution for the direct recycling of lithium iron phosphate batteries. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the lithium iron phosphate hydrothermal lithium replenishment-brief sintering direct regeneration process provided in Embodiment 1 of the present invention.

[0019] Figure 2 The image shows a comparison of the X-ray diffraction (XRD) spectra of the commercial new material, the waste degraded material, and the recycled material in Example 1, indicating that the surface phase structure of the recycled material is well restored.

[0020] Figure 3 The image shown is a high-resolution transmission electron microscope (HRTEM) image of the recycled material from Example 1, which clearly shows a uniformly coated carbon layer on the material surface.

[0021] Figure 4 The graph shows a comparison of the initial charge-discharge curves of the three materials in Example 1, indicating that the charge-discharge platform and capacity of the recycled materials have been restored to commercial levels.

[0022] Figure 5 The graph shows a comparison of the long-cycle performance of the three materials in Example 1 at a rate of 0.33C, demonstrating that the recycled material has excellent cycle stability. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0024] Example 1

[0025] A method for direct recovery and repair of LFP via hydrothermal lithium replenishment and brief sintering, such as... Figure 1 As shown, the specific steps are as follows:

[0026] (1) Take a positive electrode sheet from a retired power lithium iron phosphate battery, cut it, immerse it in NMP solvent, and heat it at 120°C for 12 hours to fully dissolve the PVDF binder. After filtration, washing, and drying, about 500 mg of degraded lithium iron phosphate (DLFP) powder is obtained.

[0027] (2) Add 500 mg of DLFP powder and 80 mL of mixed solution (containing 0.2 M LiOH·H2O and 0.08 M citric acid) to a 100 mL polytetrafluoroethylene-lined hydrothermal reactor, seal it, and place it in an oven. React at 100 °C for 4 hours. After the reaction is complete, allow it to cool naturally, wash the product repeatedly with deionized water until neutral, and dry it in a vacuum drying oven at 80 °C for 12 hours to obtain the intermediate for lithium replenishment and repair.

[0028] (3) Weigh the above intermediate and add 4% by weight of sucrose as a carbon source, and put them together into a ball mill jar. Use zirconia balls as the grinding medium and ball mill at 400 rpm for 4 hours to ensure uniform adhesion of the carbon source.

[0029] (4) The ball-milled mixture was transferred to a tube furnace and heated to 700°C at a heating rate of 5°C / min under continuous flow of high-purity nitrogen (N2) protection. It was then held at this temperature for sintering for 10 hours. Subsequently, the mixture was cooled to room temperature under controlled conditions to obtain the final regenerated lithium iron phosphate cathode material (RLFP).

[0030] Examples 2-5 and Comparative Examples 1-6

[0031] To investigate the effects of various process parameters, following the basic process of Example 1, the hydrothermal temperature, carbon source type, carbon source addition amount, ball milling time, and sintering temperature were adjusted respectively. Specific parameters and corresponding material property test results are detailed in the table below:

[0032]

[0033] Material characterization: X-ray diffraction (XRD) was used to analyze the crystal structure, and the results showed that all recycled materials maintained a standard olivine structure without impurities. Transmission electron microscopy (TEM) was also used to characterize the crystal structure. Figure 3 It can be observed that the surface of the recycled material is completely covered by a uniform, continuous amorphous carbon layer, with a thickness of approximately 3-5 nm. (XRD pattern) Figure 2 The results showed that the surface of the recycled material was completely transformed into the lithium iron phosphate phase, indicating the successful elimination of the iron phosphate phase in the degraded material.

[0034] Electrochemical testing: Recycled materials, commercially available new materials (CLFP), and unrepaired degraded materials (DLFP) were used as positive electrode active materials, respectively. These were mixed with conductive carbon black and PVDF at a mass ratio of 8:1:1 to form a slurry, which was then coated onto aluminum foil to prepare a coin cell (CR2032) for testing. The electrolyte was a 1M LiPF6 EC / DEC (1:1 vol%) solution, and the counter electrode was lithium metal.

[0035] First charge-discharge test: conducted at a rate of 0.1C (1C = 170 mA / g). Results ( Figure 4The results show that the initial discharge capacity of the recycled material (RLFP) in Example 1 is 156.71 mAh / g, which is almost the same as that of CLFP (157.50 mAh / g) and much higher than that of DLFP (120.32 mAh / g), and the charge and discharge plateau voltage is completely consistent with that of CLFP.

[0036] Cyclic performance testing: Long-cycle testing was conducted at a 0.33C rate. Results ( Figure 5 The results show that the capacity retention rate of the recycled material in Example 1 is as high as 98% after 100 cycles, demonstrating excellent cycle stability comparable to commercial materials.

[0037] Data Analysis and Conclusions

[0038] Based on the comparison of data from the embodiments and comparative examples in Table 1, the following optimization conclusions can be drawn:

[0039] Hydrothermal temperature: When the temperature is below 100℃ (e.g., 80℃ in Comparative Example 1), lithium replenishment is insufficient and performance recovery is poor; 100-120℃ is the optimal range.

[0040] Carbon source and dosage: Sucrose, glucose, and dopamine can all be used as effective carbon sources. There is an optimal window for the amount of carbon source added; too low (2%) or too high (6%) will lead to uneven coating or excessive thickness, affecting lithium-ion diffusion. The optimal addition amount is 4-5 wt%.

[0041] Ball milling time: Too short a ball milling time (such as comparative example 4, 2h) cannot achieve uniform dispersion and adhesion of carbon source, resulting in uneven carbon coating and performance degradation; ball milling for 4-6 hours yields the best results.

[0042] Sintering temperature: The sintering temperature directly affects the graphitization degree and density of the carbon layer. If the temperature is too low (600℃), carbonization will be incomplete and the conductivity will be poor; if the temperature is too high (800℃), LFP may decompose or the carbon layer structure may be destroyed. 700℃ is the optimal temperature for achieving high-performance carbon coating.

[0043] The technical scope of this invention is not limited to the content described above. Those skilled in the art can make various modifications and variations to the above embodiments without departing from the technical concept of this invention, and all such modifications and variations should fall within the protection scope of this invention.

Claims

1. A highly efficient regeneration method for waste lithium iron phosphate cathode materials, characterized in that, Includes the following steps: (1) The capacity-degraded cathode material was separated from the retired lithium iron phosphate battery, and its surface carbon coating layer was damaged. (2) The degraded cathode material is subjected to lattice lithium replenishment and structural repair with lithium-containing compounds and organic reducing agents under hydrothermal conditions to restore its lithium stoichiometry and crystal integrity. (3) The repair material obtained in step (2) is uniformly mixed with a functional carbon source, and then subjected to low-temperature short-term sintering in an inert atmosphere to pyrolyze the carbon source and form a continuous and uniform conductive carbon coating layer on the surface of the material. This allows for the simultaneous replenishment of bulk lithium and reconstruction of the surface conductive network, resulting in regenerated lithium iron phosphate cathode materials with electrochemical performance restored to commercial levels.

2. The method according to claim 1, characterized in that, The lithium-containing compound in step (2) is lithium hydroxide, and the organic reducing agent is citric acid; the temperature of the hydrothermal reaction is 100-120°C, and the reaction time is 3-5 hours.

3. The method according to claim 1, characterized in that, The functional carbon source mentioned in step (3) is selected from one or more of sucrose, glucose, and dopamine.

4. The method according to claim 3, characterized in that, The amount of the functional carbon source added is 4-5 wt% of the mass of the decaying cathode material.

5. The method according to claim 1, characterized in that, The mixing method described in step (3) is ball milling, with a milling time of 4 to 6 hours and a rotation speed of 350 to 450 rpm.

6. The method according to claim 1, characterized in that, The temperature for the short-term sintering in step (3) is 650-750°C, the sintering time is 9-11 hours, and the atmosphere is nitrogen.

7. The method according to claim 6, characterized in that, The sintering temperature is 700℃.

8. A recycled lithium iron phosphate cathode material, characterized in that, It is prepared by any one of claims 1 to 7, and its initial discharge capacity is ≥155mAh / g, and its capacity retention rate is ≥97% after 100 cycles.

9. A lithium-ion battery positive electrode, characterized in that, It includes a current collector, a binder, and the recycled lithium iron phosphate cathode material as described in claim 8.

10. A lithium-ion battery, characterized in that, It includes the positive electrode as described in claim 9 and is suitable for use in the fields of power batteries or energy storage batteries.

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