A subcritical hydrothermal repair system and method for retired lithium iron phosphate battery positive electrode material

Abstract

The application discloses a subcritical hydrothermal repair system and method for retired lithium iron phosphate battery positive electrode materials, and belongs to the technical field of lithium ion battery recycling. 3+ In view of the defects of the existing industrialized repair system for the retired lithium iron phosphate positive electrode materials, the application proposes that the retired lithium iron phosphate battery positive electrode materials are simultaneously subjected to lattice lithium supplement, Fe 3+ reduction to Fe 2+ and in-situ carbon coating in the subcritical reaction and repair unit, so that the repair products are obtained. The system can also differentially regulate the carbon source addition amount and the reaction time according to the negative electrode graphite content of the retired lithium iron phosphate battery positive electrode materials, so that the repair products are used as lithium iron phosphate positive electrode materials, positive electrode pre-lithiation additives or positive electrode composite conductive agents. The system has the advantages of short process flow, low energy consumption, good environmental protection and excellent electrochemical performance of the products, and realizes green, short-flow and high-value repair and regeneration of the retired lithium iron phosphate positive electrode materials.

Description

Technical Field

[0001] This invention belongs to the field of waste lithium-ion battery recycling and cathode material regeneration technology, specifically involving a subcritical hydrothermal repair system and method for retired lithium iron phosphate battery cathode materials. It is a green and short-process regeneration method for retired lithium iron phosphate (LiFePO4, abbreviated as LFP) battery cathode materials. Background Technology

[0002] With the rapid development of electric vehicles and energy storage systems, lithium iron phosphate (LFP) batteries have seen a continuous increase in their installed capacity due to their high safety, long cycle life and low cost. Consequently, the number of retired batteries has increased dramatically. However, early-installed lithium-ion batteries are now facing a peak in retirement (5 to 8 years of lifespan).

[0003] In decommissioned LFP cathode materials, lithium is often lost (lithium deficiency can reach 5%~20%), and some Fe is lost as well. 2+ Oxidized to Fe 3+ This leads to capacity decay and structural defects. Currently, the hydrometallurgical route requires acid leaching-precipitation-extraction separation. The core of this route is to dissolve metal ions into the liquid phase and then separate and purify them. Acid leaching involves mixing black powder with an acid solution (sulfuric acid, hydrochloric acid, etc.) and adding a reducing agent (hydrogen peroxide, sodium sulfite, etc.) to remove Fe. 3+ Reduced to Fe 2+ The process involves heating and stirring to dissolve lithium, iron, and phosphorus into the liquid phase, resulting in a leachate and a leaching residue. Alkali (NaOH) is added to the leachate to adjust the pH, thereby precipitating and removing impurities such as aluminum and copper. Alternatively, iron can be separated by oxidation (adding H2O2) to precipitate as FePO4·2H2O. The operation process is lengthy (8 to 12 steps), consumes a lot of acid and alkali, and generates a large amount of wastewater (5 to 10 tons of wastewater per ton of waste). From an equipment perspective, the equipment must withstand the "double attack" of strong acid and alkali. To resist this strong corrosion, the factory must use expensive acid-resistant alloys (such as Hastelloy and duplex stainless steel) or anti-corrosion lining materials (such as PTFE, ceramic, and rubber lining). This not only significantly increases the initial fixed asset investment, but also causes the anti-corrosion layer to wear out quickly, resulting in extremely high maintenance and replacement costs. This series mode of "one pot for one sedimentation and one pot for one filtration" leads to an exponential increase in the number of equipment. A large amount of wastewater (5 to 10 tons per ton of waste) ultimately needs to be evaporated and crystallized by a multi-effect evaporator or MVR (mechanical vapor recompression) system. In summary, this wet process is essentially an inefficient physical process that uses expensive, corrosion-resistant equipment to treat large amounts of useless water and separates impurities through repeated heating, chemical addition, and filtration. Its drawbacks are: huge equipment investment, extremely high maintenance costs, large land area required, staggering energy consumption, and difficulty in achieving truly continuous and stable operation.

[0004] The pyrometallurgical route requires high-temperature smelting / roasting. The core of this route is to mix electrode sheets or black powder with flux (such as coke, Na2CO3, NaOH, etc.) evenly, and then carry out reduction roasting or oxidation roasting in a high-temperature furnace (such as rotary kiln or electric arc furnace) at 1000℃~1500℃. The roasted slag is then crushed and finely ground, and the metal alloy or iron oxide is separated by magnetic separation or sieving. This operation has extremely high energy consumption (>1200℃) and low lithium recovery rate (usually below 80%). From an equipment perspective, the high-temperature furnace lining needs to withstand continuous heat loads above 1200℃, and the melt / slag has a strong alkaline (Na2CO3, NaOH flux) and reducing atmosphere. The replacement cycle of refractory materials (magnesia-chrome bricks, high-alumina bricks) is shortened from several years in ordinary smelting to several months or even several weeks. High-temperature furnaces all require a large number of water cooling systems and special heat-resistant steel components, resulting in high equipment costs. Crushing / fine grinding equipment suffers severe wear. The calcined sand has a complex composition, and the magnetic separation equipment is not efficient in processing complex calcined sand. In summary, the pyrometallurgical route involves extreme high temperature and strong alkaline corrosion in equipment, leading to rapid wear and tear of refractory materials, huge investment in high-temperature furnaces, severe crushing wear, and poor magnetic separation efficiency.

[0005] The high-temperature solid-phase direct repair route requires a process of repair followed by carbon coating. The core of this route involves calcining the waste electrode sheets at 300℃~700℃ in air or an inert atmosphere to remove PVDF binder and residual organic matter. The Li / Fe molar ratio in the waste powder is then measured, and a lithium source (Li₂CO₃, etc.) is added according to a stoichiometric ratio (e.g., Li:Fe = 1.05:1). A small amount of reducing agent (such as carbon powder, melamine, etc.) is also added to remove the Fe. 3+ Reduced to Fe 2+The mixture is ball-milled evenly in an agate jar, and pre-fired at 350°C to remove volatiles under an inert / reducing atmosphere (argon, hydrogen-argon mixture). The temperature is then raised to 600°C~700°C and held to allow lithium ions to re-intercalate into the crystal lattice. The repaired LFP powder is then mixed / ball-milled again with an organic carbon source (glucose, sucrose, etc.), dried, and then subjected to high-temperature carbonization sintering again under an inert atmosphere (e.g., 600°C~700°C) to form a conductive carbon coating layer. This process involves separating the repair and carbon coating steps, resulting in high energy consumption and weak carbon layer bonding and poor uniformity. From an equipment perspective, the impact of separating the process (repair and carbon coating) is not essentially an extra step, but rather that it breaks down the complete life cycle of a sintering furnace into two cycles: furnace start-up, charging, heating, holding, cooling, and discharging. This doubles the number of equipment, halves the utilization rate of a single unit, adds an entire chain of mixing / anti-oxidation / drying equipment, causes two furnaces to wait for each other leading to a capacity bottleneck, and doubles all equipment losses related to furnace start-up (thermal shock, sealing, gas consumption). Furthermore, the two-step method separates the repair and carbon coating, effectively locking the "carbon source can only be added externally + static sintering" route in terms of equipment – ​​ball mills cannot provide molecular-level uniform coating, and tube furnaces / box furnaces cannot provide a gradient-free temperature control and agglomeration-free sintering environment. More importantly, in terms of mass transfer, the equipment cannot provide a uniform molecular-level distribution of the three materials; in terms of temperature control / atmosphere, the equipment cannot provide an optimal temperature window overlap for lithium diffusion and carbon coating; and in terms of process timing, it is locked by the "detect first, then coat with carbon" operational logic. These three limitations of the equipment's capabilities make it physically impossible to achieve lithium replenishment + Fe reduction. 3+ The dual-lattice repair and conductive network construction were completed simultaneously. Summary of the Invention

[0006] To address the shortcomings of existing industrial-scale remediation systems for retired lithium iron phosphate battery cathode materials, such as lengthy processes, high energy consumption, and inability to simultaneously reduce Fe, 3+ Addressing the core issue of reconstructing the conductive network, this invention proposes a subcritical hydrothermal repair system for retired lithium iron phosphate battery cathode materials. This invention aims to simultaneously complete the re-intercalation of lithium into the lithium-deficient lattice and the oxidation of Fe in a closed reaction system using an ultra-short process, low temperature, and green method. 3+ Reduced to Fe 2+ The in-situ uniform strong bonding reconstruction of the surface conductive carbon layer overcomes the separation problem of existing equipment processes, thereby achieving efficient repair of failed materials.

[0007] To achieve this technical objective, the technical solution of the present invention is as follows: This invention protects a subcritical hydrothermal repair system for retired lithium iron phosphate battery cathode materials, comprising a subcritical reaction and repair unit, the subcritical reaction and repair unit including: The slurry preparation module is used to mix retired lithium iron phosphate battery cathode materials with water to obtain a mixed slurry; the retired lithium iron phosphate battery cathode materials and water are mixed at a solid-liquid ratio of 1:20g / mL to 50g / mL.

[0008] The lithium source and reducing carbon source addition module is used to supplement the slurry preparation module with lithium source and / or reducing carbon source precursor according to the composition of the mixed slurry, so as to obtain black powder slurry after supplementation. The lithium source and reducing carbon source addition module includes a reducing carbon source addition module and a lithium source supplementation module. The lithium source consists of an external lithium source and a recovered lithium source obtained from the mother liquor recovery module and the lithium recovery module. By adding the above two lithium sources in combination, a closed-loop cycle of lithium resources in the reaction system is formed, which achieves the effect of reducing the amount of fresh lithium salt added, reducing raw material costs and maximizing resources.

[0009] The temperature-controlled conveying module is used to pre-temperature control the black powder slurry, creating near-subcritical hydrothermal conditions in advance to ensure stable reaction system parameters and continuous material conveying.

[0010] The subcritical reaction module is used to simultaneously complete lattice lithium replenishment and Fe reaction of materials. 3+ Reduced to Fe 2+ and in-situ carbon coating; this process is driven by reducing intermediates generated from the hydrothermal carbonization of reducing carbon source precursors, which in turn drive Fe. 3+ The reduction and carbonization process simultaneously creates amorphous carbon that uniformly coats the particle surface. Lithium ions then rapidly embed into the vacancy in the lithium iron phosphate lattice with the help of the high diffusion rate of subcritical water. This achieves the integration of defect repair and conductive network construction, avoiding the need for multiple reactors and intermediate separation steps required for lithium replenishment, reduction, and coating in traditional multi-step processes. This significantly simplifies the process, shortens the processing time, and reduces energy consumption and equipment investment.

[0011] The solid-liquid separation module is used to separate the reaction product into a solid repair product and a mother liquor. The repair product is LiFePO4 / C composite powder.

[0012] The redox control module is used to replenish hydrogen peroxide in the subcritical reaction module and to regulate the redox atmosphere within the module in real time, so as to achieve a process sequence of reduction followed by oxidation, i.e., first reducing Fe... 3+ Reduced to Fe 2+ Then, excess organic matter (residual organic matter in black powder, incompletely carbonized glucose) and volatile gases are oxidized and removed. The amount of hydrogen peroxide added is 0.6 to 1.5 times that of the reducing carbon source precursor.

[0013] The slurry preparation module, temperature control and conveying module, subcritical reaction module and solid-liquid separation module are sequentially connected; the lithium source and reducing carbon source addition module are connected to the slurry preparation module; the redox control module is connected to the subcritical reaction module.

[0014] Preferably, the lithium source is selected from one or more of lithium hydroxide, lithium acetate, and lithium carbonate. The amount of lithium source added is determined based on the lithium and iron content in the mixed slurry, so that the ratio of the total molar amount of lithium in the mixed slurry (original and added) to the molar amount of iron is 1.02~1.20:1. This ratio range ensures sufficient repair of lattice lithium vacancies while avoiding the formation of impurity phases such as Li3PO4 on the particle surface due to excessive lithium source, thereby ensuring the purity and electrochemical performance of the repaired material.

[0015] Preferably, the reducing carbon source precursor is selected from one or more of glucose, sucrose, fructose, ascorbic acid, and citric acid; the amount of reducing carbon source precursor added is 0% to 20% of the mass of black powder. The amount of reducing carbon source precursor added is adjusted according to the type of cathode material of retired lithium iron phosphate batteries to avoid excessive coating and reduce process costs.

[0016] Preferably, the temperature in the temperature-controlled conveying module is pre-adjusted to 130℃~170℃.

[0017] Preferably, within the subcritical reaction module, lattice lithium replenishment and Fe2O3 production are completed simultaneously. 3+ Reduced to Fe 2+ The conditions for in-situ carbon coating are: reaction at 150℃~350℃ for 0.5h~6h.

[0018] Preferably, the subcritical reaction and remediation unit is further connected to a pretreatment and fractionation unit, which includes: Raw material storage module, which is used to store retired lithium iron phosphate batteries.

[0019] The pre-processing module is used to sequentially discharge, disassemble, and separate retired lithium iron phosphate batteries.

[0020] The raw material selection module categorizes retired lithium iron phosphate battery cathode materials into three types based on their negative electrode graphite content: Type A is pure cathode powder (i.e., lithium iron phosphate cathode powder obtained from the cathode sheet); Type B is cathode powder doped with 5wt%~10wt% negative electrode graphite (i.e., cathode powder after flotation and significant separation of negative electrode graphite); and Type C is a mixed powder of retired lithium-ion battery cathode and negative electrode materials (the black powder obtained from the mechanical pretreatment process of whole batteries widely used in industry, typically a mixture of cathode active material and negative electrode graphite). In Type C, the graphite mass percentage is 20wt%-50wt%. This invention's method can be applied to retired lithium iron phosphate battery cathode materials with varying graphite contents in existing technologies. This classification allows for subsequent selective adjustment of the amount of reducing carbon source precursor and reaction conditions based on the differences in graphite content in various materials, achieving precise remediation and resource conservation.

[0021] The raw material storage module, pretreatment module, and raw material selection module are connected in sequence. The outlet of the raw material storage module is connected to the inlet of the pretreatment module, the outlet of the pretreatment module is connected to the inlet of the raw material selection module, the slurry preparation module is connected to the raw material selection module, and the inlet of the slurry preparation module is connected to the outlet of the raw material selection module.

[0022] Preferably, the subcritical reaction and remediation unit is further connected to a post-processing unit, which includes: The system includes: a drying module for drying the repair product; a finished product recovery module for testing the dried LiFePO4 / C composite powder; a mother liquor recovery module for collecting the mother liquor after solid-liquid separation; a precipitant addition module for adding a precipitant to the mother liquor, which reacts with lithium to form a lithium salt solid; a lithium recovery module for returning the lithium salt solid as a supplementary lithium source to the lithium source replenishment module for recycling; and a residual gas collection module for collecting volatile gases such as methane generated during the reaction.

[0023] The solid-liquid separation module is connected to the drying module, the mother liquor recovery module, and the residual gas collection module. The finished product recovery module is connected to the drying module. The precipitant addition module and the lithium recovery module are connected to the mother liquor recovery module. The lithium recovery module is also connected to the lithium source replenishment module.

[0024] Preferably, the precipitant is selected from one or more of sodium carbonate, sodium phosphate, or sodium hydroxide, and the recovered product is lithium carbonate or lithium phosphate. Through this precipitation and recovery step, the residual lithium resources in the mother liquor are effectively recovered, and the recovered product can be returned to the lithium source replenishment module for recycling as a supplementary lithium source, thereby forming a closed-loop cycle of lithium resources within the reaction system and reducing the consumption of fresh lithium salts.

[0025] Preferably, the finished product recycling module uses the solid products as lithium iron phosphate cathode materials, cathode pre-lithiation additives, or cathode composite conductive agents according to their physicochemical properties. In the finished product recycling module, when the Li / Fe molar ratio of the repaired product is 0.98~1.05:1 and the carbon content is 1%~5%, it is initially diverted as a candidate product for lithium iron phosphate cathode materials; when the Li / Fe molar ratio of the repaired product is 1.05~1.20:1, it is initially diverted as a candidate product for lithium-containing composite lithium replenishment; when the carbon content of the repaired product is ≥20%, it is initially diverted as a candidate product for lithium iron phosphate / graphite composite conductive agents. After each candidate product passes the corresponding phase, chemical composition, powder performance, and electrochemical performance tests, its final application direction is determined.

[0026] This invention also protects a method for a subcritical hydrothermal repair system for retired lithium iron phosphate battery cathode materials, comprising the following steps: The retired lithium iron phosphate batteries in the raw material storage module are transported to the pretreatment module. After being discharged, disassembled and separated in the pretreatment module, the positive electrode material of the retired lithium iron phosphate batteries is obtained. The positive electrode material of the retired lithium iron phosphate batteries is classified according to the graphite content of the negative electrode in the raw material selection module.

[0027] The sorted retired lithium iron phosphate battery cathode materials are transported to the slurry preparation module. Lithium source and reducing carbon source precursors are added to the slurry preparation module via the lithium source and reducing carbon source addition module. In the slurry preparation module, the retired lithium iron phosphate battery cathode materials are mixed with water, lithium source, and reducing carbon source precursors to obtain the final material. This material is then transported to the temperature-regulating conveying module for pre-temperature regulation treatment, and then to the subcritical reaction module to simultaneously complete lattice lithium replenishment and Fe... 3+ Reduced to Fe 2+ In addition to in-situ carbon coating, the redox control module replenishes hydrogen peroxide into the subcritical reaction module to regulate the redox atmosphere in the subcritical reaction module in real time. After the reaction is completed, it is transported to the solid-liquid separation module, and the reaction product is separated into solid repair product and mother liquor. At the same time, volatile gases such as methane flowing in the solid-liquid separation module are collected by the residual gas collection module.

[0028] After the repair product is dried by the drying module, it is tested in the finished product recovery module. The mother liquor is collected by the mother liquor recovery module and then a precipitant is added to the mother liquor through the precipitant addition module to generate lithium salt solid. The lithium salt solid enters the lithium recovery module and is then transported to the lithium source replenishment module.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Subcritical water, with a temperature range of 150℃ to 350℃, possesses unique high-temperature, high-pressure liquid physicochemical properties, resulting in high ion product (autocatalytic reaction environment), low dielectric constant (enhanced dissolution and transport capabilities), and excellent mass transfer performance. Based on this, this invention proposes a subcritical hydrothermal repair system for retired lithium iron phosphate battery cathode materials. Utilizing the potential advantages of subcritical water, lithium replenishment, reduction, and in-situ carbon coating are achieved simultaneously in one step within the same reaction medium in a subcritical reaction module. The principle behind these processes is as follows: Under subcritical water conditions, the lithium source fully dissociates, allowing lithium ions to rapidly penetrate the lithium iron phosphate lattice and fill lithium vacancies, thus completing the lithium replenishment. A lithium source and reducing carbon source addition module supplements the subcritical reaction module with sugar-based reducing carbon source precursors such as glucose and sucrose. The pyrolysis of these precursors generates reducing groups, reducing the exhausted ferric iron to active ferrous iron. Simultaneously, the sugars undergo gradual low-temperature carbonization, forming a uniform carbon layer on the particle surface, achieving in-situ coating. Compared to existing technologies, the three-step process of existing technologies is completed simultaneously in a single-reactor reaction of this invention.

[0030] This invention addresses the shortcomings of existing decommissioned lithium iron phosphate cathode repair systems, which suffer from lengthy processes, high energy consumption, and inability to simultaneously reduce Fe. 3+ To address the shortcomings of reconstructing the conductive network, a subcritical reaction module is used to provide a subcritical hydrothermal system. In a low-temperature, closed, aqueous, green system, lithium replenishment and reconstruction of the lithium-deficient lattice are achieved simultaneously in one step, along with Fe... 3+ Directional reduction to Fe 2+ The in-situ construction of the surface carbon coating layer enabled simultaneous "three-in-one" repair, significantly simplifying system equipment investment and reducing energy consumption. Simultaneously, the design of a mother liquor recovery module facilitated the precipitation and recovery of the mother liquor, completing a closed-loop cycle of lithium resources.

[0031] 2. The system of the present invention can also perform differentiated grading: the cathode materials of retired lithium iron phosphate batteries are divided into three categories: A, B and C. For different categories, the reducing carbon source addition module and the lithium source replenishment module are selectively controlled, thereby realizing the control of the amount of reducing carbon source precursor added and the reaction time. This achieves the precise utilization of graphite-containing materials, avoids the waste or excessive coating of carbon sources, and has stronger universality.

[0032] 3. The in-situ carbon layer of the repair product prepared by the system of the present invention has excellent quality: the carbon layer is generated in-situ by hydrothermal carbonization in the subcritical reaction module, and is tightly bonded to the matrix, with uniform and controllable thickness, and has better conductivity than exogenous physical mixed carbon coating.

[0033] 4. The system provided by this invention is highly environmentally friendly and economical: the reaction temperature in the subcritical reaction module is only 150℃~350℃, which is far lower than the 600℃~800℃ required by the existing industrial high-temperature solid-phase method system, reducing energy consumption by about 75%; the system of this invention uses water as a medium, does not require strong acids or alkalis, and has almost no wastewater discharge, overcoming the defects of the existing industrial hydrometallurgical route; the lithium recovery module realizes the effective recovery of lithium resources in the mother liquor, which meets the requirements of green circular economy.

[0034] In summary, existing industrial-scale decommissioned lithium iron phosphate cathode material repair systems suffer from lengthy processes, high energy consumption, and the inability to simultaneously reduce Fe. 3+ To address the shortcomings of reconstructing the conductive network, this invention proposes to simultaneously complete lattice lithium replenishment and Fe2O3 production of retired lithium iron phosphate battery cathode materials within a subcritical reaction and repair unit. 3+ Reduced to Fe 2+ The system involves in-situ carbon coating to obtain the repaired product. Furthermore, it can classify retired lithium iron phosphate battery cathode materials based on the graphite content of the anode, and differentiate the amount of carbon source added and the reaction time, allowing the repaired product to be used as a lithium iron phosphate cathode material, a cathode pre-lithiation additive, or a cathode composite conductive agent. This invention system has the advantages of a short process flow, low energy consumption, good environmental performance, and excellent electrochemical performance of the product, achieving green, short-process, and high-value repair and regeneration of retired lithium iron phosphate cathode materials. It solves three core problems that existing technologies cannot achieve: dual repair of lattice defects (simultaneous lithium replenishment and Fe reduction). 3+ ), synchronous construction of conductive networks and extreme simplification of process flow. Attached Figure Description

[0035] Figure 1 This is a schematic diagram of the overall process flow of the subcritical hydrothermal repair method for retired lithium iron phosphate battery cathode materials according to the present invention.

[0036] Explanation of reference numerals in the attached diagram: 1. Raw material storage module; 2. Pretreatment module; 3. Raw material selection module; 4. Slurry preparation module; 5a. Reducing carbon source addition module; 5b. Lithium source replenishment module; 6. Temperature-controlled conveying module; 7. Subcritical reaction module; 8. Solid-liquid separation module; 9. Drying module; 10. Finished product recovery module; 11. Mother liquor recovery module; 12. Precipitant addition module; 13. Lithium recovery module; 14. Residual gas collection module; 15. Oxidation-reduction regulation module. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0038] Based on the differences in graphite content in the cathode materials of retired lithium iron phosphate batteries, these materials are categorized into three types for differentiated treatment. This invention's system demonstrates broad adaptability and promising industrial application prospects for cathode materials from retired batteries of different origins. The retired lithium iron phosphate cathode materials used in this invention are all derived from commercially available retired lithium iron phosphate power batteries, conforming to the standard operating conditions for waste power battery cathode materials.

[0039] This invention provides a subcritical hydrothermal repair system for retired lithium iron phosphate battery cathode materials, comprising a pretreatment and classification unit, a subcritical reaction and repair unit, and a post-treatment unit.

[0040] The pretreatment and grading unit includes a raw material storage module 1, a pretreatment module 2, and a raw material selection module 3, which are connected in sequence.

[0041] The subcritical reaction and remediation unit includes a slurry preparation module 4, a lithium source and reducing carbon source addition module, a temperature control and conveying module 6, a subcritical reaction module 7, a solid-liquid separation module 8, and a redox control module 15. The slurry preparation module 4, the temperature control and conveying module 6, the subcritical reaction module 7, and the solid-liquid separation module 8 are connected in sequence. The lithium source and reducing carbon source addition module is connected to the slurry preparation module 4. The redox control module 15 is connected to the subcritical reaction module 7. The slurry preparation module 4 is connected to the raw material selection module 3.

[0042] The post-processing unit includes a drying module 9, a finished product recovery module 10, a mother liquor recovery module 11, a precipitant addition module 12, a lithium recovery module 13, and a residual gas collection module 14. The solid-liquid separation module 8 is connected to the drying module 9, the mother liquor recovery module 11, and the residual gas collection module 14. The finished product recovery module 10 is connected to the drying module 9. The precipitant addition module 12 and the lithium recovery module 13 are connected to the mother liquor recovery module 11. The lithium recovery module 13 is also connected to the lithium source replenishment module 5b.

[0043] The present invention will be described in detail below through specific embodiments. Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods.

[0044] Example 1 This embodiment provides a subcritical hydrothermal repair method for retired lithium iron phosphate battery cathode materials, specifically using Class A retired lithium iron phosphate battery cathode materials as raw materials, and following... Figure 1 The process flow shown sequentially involves pretreatment and grading unit, subcritical reaction and repair unit, and posttreatment unit.

[0045] In the pretreatment and grading unit, commercially retired lithium iron phosphate batteries are discharged and stored in the raw material storage module 1. Then, the discharged retired lithium iron phosphate batteries are transported to the pretreatment module 2 and disassembled to remove the shell, separator, electrolyte and copper and aluminum foil current collector. After mechanical crushing, the retired lithium iron phosphate battery positive electrode material is obtained by vibrating sieve. The raw material selection module 3 is used to classify the retired lithium iron phosphate battery positive electrode material into three categories according to the graphite content of the negative electrode. In this embodiment, the Class A retired lithium iron phosphate battery positive electrode material (pure positive electrode powder) is selected as the processing object.

[0046] The cathode material of retired Class A lithium iron phosphate batteries was mixed with water at a solid-liquid ratio of 1:25 g / mL to obtain a mixed slurry. The mixed slurry was then transported to the slurry preparation module 4. A reducing carbon source precursor and a lithium source were added to the slurry preparation module 4 through a reducing carbon source addition module 5a and a lithium source replenishment module 5b. The mixture was stirred for 20 minutes to ensure the lithium source and the reducing carbon source precursor were fully dissolved and dispersed, resulting in a black powder slurry. Lithium hydroxide was used as the lithium source, with an addition amount calculated based on a target Li / Fe molar ratio of 1.02:1. Glucose was used as the reducing carbon source precursor, with an addition amount equal to 10% of the mass of the retired lithium iron phosphate battery cathode material. The mixture then entered the temperature-controlled conveying module 6 and was heated to 130℃. The heated black powder slurry was pumped to the subcritical reaction module 7 via the temperature-controlled conveying module 6. In a closed high-pressure reactor, the reaction temperature was set to 200℃, and the reaction time was 2.5 hours. Under these conditions, the material simultaneously underwent lattice lithium replenishment and Fe... 3+ Reduced to Fe 2+ The reaction involves three remediation processes: reduction, oxidation, and in-situ carbon coating. Simultaneously, during the reaction, the redox control module 15 supplies hydrogen peroxide to the subcritical reaction module 7, real-time controlling the redox atmosphere within the reactor to achieve a process sequence of reduction followed by oxidation: first, Fe... 3+ Reduced to Fe 2+ Then, excess organic matter (residual organic matter in black powder, and incompletely carbonized glucose) is oxidized and removed.

[0047] In this system, hydrogen peroxide is only used for the oxidative decomposition of unreacted carbohydrate precursors, organic degradation impurities, and binder residues. Oxidation of reduced Fe is strictly prohibited. 2+ Therefore, the dosage should be "just enough to remove organic impurities without causing secondary oxidation" as the bottom line. The addition standard is based on the mass of the reducing carbon source precursor and COD. When the carbon source is glucose, the amount of hydrogen peroxide added is 1 times the mass of glucose.

[0048] After the reaction is completed, the product slurry is naturally cooled to room temperature and transferred to the solid-liquid separation module 8 for vacuum filtration to obtain solid product and mother liquor. The solid product is sent to the post-processing unit, and the mother liquor is collected and used as lithium resources to be recovered. At the same time, the residual gas collection module 14 collects residual volatile gases such as methane to avoid their emission and environmental pollution.

[0049] The mother liquor obtained from solid-liquid separation is collected in mother liquor recovery module 11 and then transported to lithium recovery module 13. A saturated sodium carbonate solution (5% of the mother liquor volume) is added via precipitant addition module 12. After reacting for 1 hour, the mixture is filtered, washed, and dried to obtain lithium carbonate solid with a purity of 98.5%. This lithium carbonate solid can be used as a supplementary lithium source and returned to lithium source replenishment module 5b for recycling, achieving closed-loop utilization of lithium resources. The solid product is dried in drying module 9 to obtain repaired LiFePO4 / C composite powder. This powder enters finished product recovery module 10 for testing. The Li / Fe molar ratio of the repaired product is 0.98~1.05:1, and the carbon content is 1%~5%, meeting the usage conditions for lithium iron phosphate cathode materials and thus suitable for use as a cathode material in lithium-ion batteries.

[0050] Example 2 This embodiment provides a subcritical hydrothermal repair method for retired lithium iron phosphate battery cathode materials. Specifically, it uses Class B retired lithium iron phosphate battery cathode materials as raw materials. In this case, 5 wt% of negative electrode graphite is doped into the retired lithium iron phosphate battery cathode materials, and then... Figure 1 The process flow shown sequentially involves pretreatment and grading unit, subcritical reaction and repair unit, and posttreatment unit.

[0051] In the pretreatment and grading unit, commercially retired lithium iron phosphate batteries are discharged and stored in the raw material storage module 1. Then, the discharged retired lithium iron phosphate batteries are transported to the pretreatment module 2 and disassembled to remove the shell, separator, electrolyte and copper and aluminum foil current collector. After mechanical crushing, the retired lithium iron phosphate battery cathode material is obtained by vibrating sieve. The raw material selection module 3 is used to classify the retired lithium iron phosphate battery cathode material into three categories according to the graphite content of the negative electrode. In this embodiment, the B-type retired lithium iron phosphate battery cathode material is selected as the processing object.

[0052] The cathode material of retired Class B lithium iron phosphate batteries was mixed with water at a solid-liquid ratio of 1:25 g / mL to obtain a mixed slurry. The mixed slurry was then transported to the slurry preparation module 4. A reducing carbon source precursor and a lithium source were added to the slurry preparation module 4 through a reducing carbon source addition module 5a and a lithium source replenishment module 5b. The mixture was stirred for 20 minutes to ensure the lithium source and the reducing carbon source precursor were fully dissolved and dispersed, resulting in a black powder slurry. Lithium hydroxide was used as the lithium source, with an addition amount calculated based on a target Li / Fe molar ratio of 1.04:1. Glucose was used as the reducing carbon source precursor, with an addition amount of 8% of the mass of the retired lithium iron phosphate battery cathode material. The mixture then entered the temperature-controlled conveying module 6 and was heated to 130℃. The heated black powder slurry was pumped to the subcritical reaction module 7 via the conveying module 6. In a closed high-pressure reactor, the reaction temperature was set to 200℃, and the reaction time was 2.5 hours. Under these conditions, the material simultaneously underwent lattice lithium replenishment and Fe... 3+ Reduced to Fe 2+ The reaction involves three remediation processes: reduction, oxidation, and in-situ carbon coating. Simultaneously, during the reaction, the redox control module 15 supplies hydrogen peroxide to the subcritical reaction module 7, real-time controlling the redox atmosphere within the reactor to achieve a process sequence of reduction followed by oxidation: first, Fe... 3+ Reduced to Fe 2+ Then, excess organic matter (residual organic matter in black powder, and incompletely carbonized glucose) is oxidized and removed.

[0053] In this system, hydrogen peroxide is only used for the oxidative decomposition of unreacted carbohydrate precursors, organic degradation impurities, and binder residues. Oxidation of reduced Fe is strictly prohibited. 2+ Therefore, the dosage should be "just enough to remove organic impurities without causing secondary oxidation" as the bottom line. The addition standard is based on the mass of the reducing carbon source precursor and COD. When the carbon source is glucose, the amount of hydrogen peroxide added is 1.2 times the mass of glucose.

[0054] After the reaction is completed, the product slurry is naturally cooled to room temperature and transferred to the solid-liquid separation module 8 for vacuum filtration to obtain solid product and mother liquor. The solid product is sent to the post-processing unit, and the mother liquor is collected and used as lithium resources to be recovered. At the same time, the residual gas collection module 14 collects residual volatile gases such as methane to avoid their emission and environmental pollution.

[0055] The mother liquor obtained from solid-liquid separation is collected in mother liquor recovery module 11 and then transported to lithium recovery module 13. A saturated sodium carbonate solution (5% of the mother liquor volume) is added via precipitant addition module 12. After reacting for 1 hour, the mixture is filtered, washed, and dried to obtain solid lithium carbonate. This solid lithium carbonate can be used as a supplementary lithium source and returned to lithium source replenishment module 5b for recycling, achieving closed-loop utilization of lithium resources. The solid product is dried in drying module 9 to obtain repaired LiFePO4 / C composite powder. This powder enters finished product recovery module 10 for testing. The Li / Fe molar ratio of the repaired product is 1.05~1.20:1, and it is initially separated into lithium-containing composite supplementary lithium product candidates.

[0056] Example 3 This embodiment provides a subcritical hydrothermal repair method for retired lithium iron phosphate battery cathode materials. Specifically, it uses Class C retired lithium iron phosphate battery cathode materials as raw materials. In this case, 40 wt% of negative electrode graphite is doped into the retired lithium iron phosphate battery cathode materials, and then... Figure 1 The process flow shown sequentially involves pretreatment and grading unit, subcritical reaction and repair unit, and posttreatment unit.

[0057] In the pretreatment and grading unit, commercially retired lithium iron phosphate batteries are discharged and stored in the raw material storage module 1. Then, the discharged retired lithium iron phosphate batteries are transported to the pretreatment module 2 and disassembled to remove the shell, separator, electrolyte and copper and aluminum foil current collector. After mechanical crushing, the retired lithium iron phosphate battery positive electrode material is obtained by vibrating sieve. The raw material selection module 3 is used to classify the retired lithium iron phosphate battery positive electrode material into three categories according to the graphite content of the negative electrode. In this embodiment, the C-type retired lithium iron phosphate battery black powder material is selected as the processing object.

[0058] Class C retired lithium iron phosphate battery black powder material was mixed with water at a solid-liquid ratio of 1:25 g / mL to obtain a mixed slurry. The mixed slurry was then conveyed to the slurry preparation module 4. A reducing carbon source precursor and a lithium source were added to the slurry preparation module 4 through a reducing carbon source addition module 5a and a lithium source replenishment module 5b. The mixture was stirred for 20 minutes to ensure the lithium source and reducing carbon source precursor were fully dissolved and dispersed, resulting in a black powder slurry. Glucose was used as the reducing carbon source precursor, and its addition amount was 7% of the black powder material mass. The mixture then entered the temperature-controlled conveying module 6 and was heated to 130℃. The mixed slurry was then pumped to the subcritical reaction module 7 via the conveying module 6. In a closed high-pressure reactor, the reaction temperature was set to 210℃, and the reaction time was 3 hours. Under these conditions, the material underwent Fe... 3+ Reduced to Fe 2+ The process involves two remediation processes: in-situ carbon coating and oxidation. Simultaneously, during the reaction, the redox control module 15 supplies hydrogen peroxide to the subcritical reaction module 7, regulating the redox atmosphere within the reactor in real time to achieve a process sequence of reduction followed by oxidation: first, Fe...3+ Reduced to Fe 2+ Then, excess organic matter (residual organic matter in black powder, and incompletely carbonized glucose) is oxidized and removed.

[0059] In this system, hydrogen peroxide is only used for the oxidative decomposition of unreacted carbohydrate precursors, organic degradation impurities, and binder residues. Oxidation of reduced Fe is strictly prohibited. 2+ Therefore, the dosage should be "just enough to remove organic impurities without causing secondary oxidation" as the bottom line. The addition standard is based on the mass of the reducing carbon source precursor and COD. When the carbon source is glucose, the amount of hydrogen peroxide added is 0.9 times the mass of glucose.

[0060] After the reaction is completed, the product slurry is naturally cooled to room temperature and transferred to the solid-liquid separation module 8 for vacuum filtration to obtain solid product and mother liquor. The solid product is sent to the post-processing unit, and the mother liquor is collected and used as lithium resources to be recovered. At the same time, the residual gas collection module 14 collects residual volatile gases such as methane to avoid their emission and environmental pollution.

[0061] The mother liquor obtained from solid-liquid separation is collected in mother liquor recovery module 11 and then transported to lithium recovery module 13. A saturated sodium carbonate solution (5% of the mother liquor volume) is added via precipitant addition module 12. After reacting for 1 hour, the mixture is filtered, washed, and dried to obtain solid lithium carbonate. This solid lithium carbonate can be used as a supplementary lithium source and returned to lithium source replenishment module 5b for recycling, achieving closed-loop utilization of lithium resources. The solid product is dried in drying module 9 to obtain repaired LiFePO4 / C composite powder. This powder enters finished product recovery module 10 for testing. If the carbon content of the repaired product is ≥20%, it is initially segregated into lithium iron phosphate / graphite composite conductive candidate products.

[0062] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A subcritical hydrothermal repair system for retired lithium iron phosphate battery cathode materials, characterized in that, It includes a subcritical reaction and repair unit, which comprises a slurry preparation module (4), a lithium source and reducing carbon source addition module, a temperature control and delivery module (6), a subcritical reaction module (7), a solid-liquid separation module (8), and a redox control module (15). The slurry preparation module (4), the temperature control and conveying module (6), the subcritical reaction module (7), and the solid-liquid separation module (8) are sequentially connected in series. The lithium source and reducing carbon source addition module is connected in series with the slurry preparation module (4). The redox control module (15) is connected in series with the subcritical reaction module (7). The slurry preparation module (4) is used to mix the cathode material of retired lithium iron phosphate batteries with water to obtain a mixed slurry; The lithium source and reducing carbon source addition module is used to supplement the slurry preparation module (4) with lithium source and / or reducing carbon source precursor according to the composition of the mixed slurry, and after supplementation, black powder slurry is obtained. The lithium source and reducing carbon source addition module includes a reducing carbon source addition module (5a) and a lithium source supplementation module (5b). The temperature-regulating conveying module (6) is used to pre-regulate the temperature of the black powder slurry to create working conditions close to subcritical hydrothermal conditions in advance. The subcritical reaction module (7) is used to bring the pre-temperature-treated black powder slurry to a subcritical state and simultaneously complete lattice lithium replenishment and Fe... 3+ Reduced to Fe 2+ and in-situ carbon coating; The solid-liquid separation module (8) is used to separate the reaction product into a solid repair product and a mother liquor. The repair product is LiFePO4 / C composite powder. The oxidation-reduction control module (15) is used to supplement hydrogen peroxide into the subcritical reaction module (7) and regulate the oxidation-reduction atmosphere in the subcritical reaction module (7) in real time to achieve the process sequence of reduction followed by oxidation.

2. The subcritical hydrothermal repair system for retired lithium iron phosphate battery cathode materials according to claim 1, characterized in that, The lithium source is selected from one or more of lithium hydroxide, lithium acetate, and lithium carbonate. The amount of lithium source added is determined according to the lithium and iron content in the mixed slurry, so that the ratio of the total molar amount of lithium in the mixed slurry (original lithium and lithium provided by the added lithium source) to the molar amount of iron is 1.02~1.20:

1.

3. The subcritical hydrothermal repair system for retired lithium iron phosphate battery cathode materials according to claim 2, characterized in that, The reducing carbon source precursor is selected from one or more of glucose, sucrose, fructose, ascorbic acid, and citric acid; the amount of the reducing carbon source precursor added is 0% to 20% of the mass of the black powder, and the black powder is the total amount of the reducing carbon source precursor, lithium source, and retired lithium iron phosphate battery cathode material.

4. The subcritical hydrothermal repair system for retired lithium iron phosphate battery cathode materials according to claim 3, characterized in that, The temperature is pre-adjusted to 130℃~170℃ in the temperature control and conveying module (6).

5. The subcritical hydrothermal repair system for retired lithium iron phosphate battery cathode materials according to claim 4, characterized in that, Within the subcritical reaction module (7), lattice lithium replenishment and Fe2O3 reaction are simultaneously completed. 3+ Reduced to Fe 2+ The conditions for in-situ carbon coating are: reaction at 150℃~350℃ for 0.5h~6h.

6. The subcritical hydrothermal repair system for retired lithium iron phosphate battery cathode materials according to claim 5, characterized in that, The subcritical reaction and remediation unit is further connected to a pretreatment and classification unit, which includes: Raw material storage module (1), which is used to store retired lithium iron phosphate batteries; The pretreatment module (2) is used to discharge, disassemble and separate retired lithium iron phosphate batteries in sequence; The raw material selection module (3) is used to classify the retired lithium iron phosphate battery positive electrode material into three categories according to the negative electrode graphite content: Category A is pure positive electrode powder, Category B is positive electrode powder with a negative electrode graphite content of 5wt%~10wt%, and Category C is a mixed powder of positive and negative electrodes of retired lithium-ion batteries, in which the graphite content is 20wt%-50wt%. The raw material storage module (1), the pretreatment module (2) and the raw material selection module (3) are connected in sequence, and the slurry preparation module (4) is connected in sequence with the raw material selection module (3).

7. The subcritical hydrothermal repair system for retired lithium iron phosphate battery cathode materials according to claim 6, characterized in that, The subcritical reaction and repair unit is further connected to a post-processing unit, which includes: Drying module (9), which is used to dry the repair products; The finished product recovery module (10) is used to detect the dried LiFePO4 / C composite powder; Mother liquor recovery module (11), which is used to collect mother liquor after solid-liquid separation; The precipitant addition module (12) is used to add precipitant to the mother liquor, and the precipitant reacts with lithium to generate lithium salt solid; The lithium recovery module (13) is used to return solid lithium salts as a supplementary lithium source to the lithium source replenishment module (5b) for recycling. The residual gas collection module (14) is used to collect residual volatile gases; The solid-liquid separation module (8) is connected to the drying module (9), the mother liquor recovery module (11), and the residual gas collection module (14) respectively. The finished product recovery module (10) is connected to the drying module (9). The precipitant addition module (12) and the lithium recovery module (13) are connected to the mother liquor recovery module (11) respectively. The lithium recovery module (13) is also connected to the lithium source replenishment module (5b).

8. The subcritical hydrothermal repair system for retired lithium iron phosphate battery cathode materials according to claim 7, characterized in that, The precipitant is selected from one or more of sodium carbonate, sodium phosphate, or sodium hydroxide, and the recovered product is lithium carbonate or lithium phosphate.

9. The subcritical hydrothermal repair system for retired lithium iron phosphate battery cathode materials according to claim 8, characterized in that, In the finished product recycling module (10), when the Li / Fe molar ratio of the repair product is 0.98~1.05:1 and the carbon content is 1%~5%, it is initially diverted into a candidate product for lithium iron phosphate cathode material; when the Li / Fe molar ratio of the repair product is 1.05~1.20:1, it is initially diverted into a candidate product for lithium-containing composite lithium replenishment; when the carbon content of the repair product is ≥20%, it is initially diverted into a candidate product for lithium iron phosphate / graphite composite conductive material; after each candidate product passes the corresponding phase, chemical composition, powder performance and electrochemical performance tests, its final application direction is determined.

10. A method for a subcritical hydrothermal repair system based on the cathode material of a decommissioned lithium iron phosphate battery as described in claim 9, characterized in that, Includes the following steps: The retired lithium iron phosphate batteries in the raw material storage module (1) are transported to the pretreatment module (2). After being discharged, disassembled and separated in the pretreatment module (2), the retired lithium iron phosphate battery positive electrode material is obtained. The retired lithium iron phosphate battery positive electrode material is classified in the raw material selection module (3) according to the negative electrode graphite content. The sorted retired lithium iron phosphate battery cathode materials are transported to the slurry preparation module (4). Lithium source and / or reducing carbon source precursors are added to the slurry preparation module (4) via the lithium source and reducing carbon source addition module. The retired lithium iron phosphate battery cathode materials are then mixed with water, lithium source, and / or reducing carbon source precursors in the slurry preparation module (4) to obtain the material. This material is then transported to the temperature-controlled conveying module (6) for pre-temperature control, and then transported to the subcritical reaction module (7) to simultaneously complete lattice lithium replenishment and Fe... 3+ Reduced to Fe 2+ In addition to in-situ carbon coating, the redox control module (15) replenishes hydrogen peroxide into the subcritical reaction module (7) to regulate the redox atmosphere in the subcritical reaction module (7) in real time. After the reaction is completed, it is transported to the solid-liquid separation module (8) and the reaction product is separated into solid repair product and mother liquor. The repaired product is dried by the drying module (9) and then tested in the finished product recycling module (10); After the mother liquor is collected by the mother liquor recovery module (11), a precipitant is added to the mother liquor via the precipitant addition module (12) to generate lithium salt solid. The lithium salt solid enters the lithium recovery module (13) and is then transported to the lithium source replenishment module (5b).

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