Ionic liquid assisted pyrolysis method for regeneration of graphite material of retired battery

Abstract

The invention provides an ionic liquid assisted pyrolysis method for regeneration of a graphite material of a decommissioned battery, and belongs to the technical field of recycling of the decommissioned battery, the ionic liquid assisted pyrolysis method comprises the following steps: soaking a pretreated graphite material in a composite ionic liquid, the composite ionic liquid comprises a component A and a component B, the component A comprises any one of imidazole ionic liquid, pyridine ionic liquid and ammonium salt ionic liquid, and the component B is fluorine-containing ionic liquid; pyrolyzing the soaked graphite material in an inert atmosphere; and cleaning and drying the pyrolyzed graphite material to obtain regenerated graphite. According to the method, the graphite in the retired battery is regenerated by adopting an ionic liquid assisted pyrolysis method, and the ionic liquid serving as a green and non-volatile solvent has good chemical stability and excellent dissolving property and can effectively promote pyrolytic reaction at low temperature, so that graphite damage possibly caused in the traditional high-temperature pyrolysis process is avoided.

Description

Technical Field

[0001] The present invention relates to the technical field of recycling retired batteries, and in particular to an ionic liquid-assisted pyrolysis method for regenerating retired battery graphite materials. Background Art

[0002] With the widespread application of electric vehicles, energy storage devices and various electronic products, lithium-ion batteries have become the main energy storage device. However, during long-term use, the electrical performance of the battery gradually declines and eventually reaches the retirement standard. The graphite negative electrode material in these retired batteries has a high raw material cost, good electrochemical performance and is recyclable, so its regeneration technology has become an important research direction for recycling battery resources.

[0003] Existing methods for regenerating retired battery graphite usually use mechanical grinding, chemical treatment or pyrolysis, but these methods have certain limitations. For example, traditional pyrolysis methods often cause surface structural damage to graphite materials during the regeneration process, or cannot efficiently remove harmful impurities therein. Chemical treatment methods may cause certain pollution to the environment. Therefore, there is an urgent need for a new, efficient and environmentally friendly regeneration method to improve the regeneration efficiency of retired battery graphite materials, extend their service life, and reduce resource waste. Summary of the invention

[0004] In view of the technical problems existing in the background technology, the present application provides an ionic liquid-assisted pyrolysis method for regenerating retired battery graphite materials, aiming to solve the technical problems that the surface of the graphite material is damaged and harmful impurities cannot be efficiently removed during the traditional pyrolysis process of retired battery graphite.

[0005] In a first aspect, the present application provides an ionic liquid-assisted pyrolysis method for regenerating retired battery graphite material, comprising the following steps: S1. Cleaning and removing impurities from retired battery graphite materials to obtain pretreated graphite materials; S2, soaking the pretreated graphite material with a composite ionic liquid, wherein the composite ionic liquid includes component A and component B, component A and component B are mutually soluble, component A includes any one of an imidazole ionic liquid, a pyridine ionic liquid and an ammonium salt ionic liquid, and component B is a fluorine-containing ionic liquid; S3, pyrolyzing the soaked graphite material under an inert atmosphere; S4, washing and drying the pyrolyzed graphite material to obtain regenerated graphite.

[0006] Preferably, the imidazole ionic liquid includes one or more of 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]), 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][PF4]), 1-ethyl-3-methylimidazolium bistrifluoromethanesulfonyl imide salt ([emin]TF2N), and 1-octyl-3-methylimidazolium tetrafluoroborate ([omin][BF4]); the pyridine ionic liquid is one or two of 1-ethyl-1-methylpyrrolidine tetrafluoroborate and N-butyl-N-methylpyrrolidine trifluoromethanesulfonate; and the ammonium salt ionic liquid includes tetrabutylammonium tetrafluoroborate ([N4444][BF4]).

[0007] Preferably, the fluorine-containing ionic liquid includes one or more of 1-ethyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methylimidazolium hexafluorophosphate, and tetramethylammonium hexafluorophosphate.

[0008] Preferably, component A is an imidazole ionic liquid, component B is tetramethylammonium hexafluorophosphate, and the volume ratio of component A to component B is (1-3):1.

[0009] Preferably, component A is a pyridine ionic liquid, component B is a fluorine-containing ionic liquid, and the volume ratio of component A to component B is (1-2):1.

[0010] Preferably, component A is an ammonium salt ionic liquid, component B is one or both of 1-ethyl-3-methylimidazolium hexafluorophosphate and 1-ethyl-3-methylimidazolium hexafluorophosphate, and the volume ratio of component A to component B is (1-5):1.

[0011] Preferably, in step S2, the mass ratio of the ionic liquid to the pretreated graphite material during immersion is 1:(1-3).

[0012] Preferably, in step S3, the pyrolysis temperature is 300-600° C., and the pyrolysis time is 0.5-3 h.

[0013] Preferably, in step S1, the graphite material is cleaned using one or more of deionized water, an organic solvent or an acidic solution.

[0014] Preferably, in step S4, the pyrolyzed graphite material is washed alternately with deionized water and ethanol for 2 to 3 times, and then dried at a temperature of 100 to 120° C. for 6 to 8 hours.

[0015] Different from the existing technical solutions, the beneficial effects of this application include: The present application adopts an ionic liquid-assisted pyrolysis method to regenerate graphite in retired batteries, which can achieve the following effects: (1) Restoration of graphite structure: As a green, non-volatile solvent, ionic liquid has good chemical stability and excellent solubility. It can effectively promote pyrolysis reaction at low temperature, avoiding the damage to graphite that may be caused by traditional high-temperature pyrolysis process. Ionic liquid can help remove impurities or degradation products on the surface of graphite, such as salts, solvents or other metal contaminants in the electrolyte, so that the structure of graphite can be restored. Through the pyrolysis process, the pore structure and layered structure of graphite may be improved, thereby restoring its battery performance. (2) Improvement of conductivity: Through regeneration, the conductivity of graphite is expected to be restored to a higher level, which is very important for the application of graphite in batteries. (3) Reduction of environmental pollution: By using ionic liquids instead of traditional solvents, it helps to reduce solvent volatilization or waste emissions, thereby reducing environmental pollution. (4) Improvement of recycling efficiency: The use of ionic liquids can improve the recovery rate and purity of graphite, making it more suitable for reuse in battery manufacturing and improving the overall resource recovery efficiency. (5) Strong adaptability: The method of the present application can be adapted to the regeneration of retired battery graphite in different health states, including lightly retired and severely retired graphite materials. Regardless of the degree of damage to the graphite surface, the ionic liquid can effectively optimize its surface structure and restore its original performance.

[0016] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. DETAILED DESCRIPTION

[0017] The following is a detailed description of the embodiments of the technical solution of the present application. The following embodiments are only used to more clearly illustrate the technical solution of the present application, and are therefore only used as examples, and cannot be used to limit the scope of protection of the present application.

[0018] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" in the description and claims of this application and any variations thereof are intended to cover non-exclusive inclusions.

[0019] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0020] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0021] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0022] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0023] Existing methods for regenerating retired battery graphite usually use mechanical grinding, chemical treatment or pyrolysis, but these methods have certain limitations. For example, traditional pyrolysis methods often cause surface structural damage to graphite materials during the regeneration process, or cannot efficiently remove harmful impurities therein. Chemical treatment methods may cause certain pollution to the environment. Therefore, there is an urgent need for a new, efficient and environmentally friendly regeneration method to improve the regeneration efficiency of retired battery graphite materials, extend their service life, and reduce resource waste.

[0024] In order to solve the technical problem that the surface of graphite materials is damaged and harmful impurities cannot be removed efficiently during the pyrolysis of traditional retired battery graphite, this application provides an ionic liquid-assisted pyrolysis method for regenerating retired battery graphite materials, which can not only effectively improve the recovery efficiency and electrochemical performance of graphite, but also avoid environmental pollution and graphite damage problems in traditional methods. This technology has the advantages of strong adaptability, green environmental protection, and economic efficiency, and has broad application prospects, providing a feasible solution for the recovery and regeneration of retired battery graphite.

[0025] In a first aspect, the present application provides an ionic liquid-assisted pyrolysis method for regenerating retired battery graphite material, comprising the following steps: S1. Cleaning and removing impurities from retired battery graphite materials to obtain pretreated graphite materials; S2, soaking the pretreated graphite material with a composite ionic liquid, wherein the composite ionic liquid includes component A and component B, component A and component B are mutually soluble, component A includes any one of an imidazole ionic liquid, a pyridine ionic liquid and an ammonium salt ionic liquid, and component B is a fluorine-containing ionic liquid; S3, pyrolyzing the soaked graphite material under an inert atmosphere; S4, washing and drying the pyrolyzed graphite material to obtain regenerated graphite.

[0026] In the technical solution of the embodiment of the present application, the graphite material of retired batteries is usually affected by multiple factors during the use of the battery, resulting in changes in its structure. Graphite before recovery may have the following problems: (1) Adhesion of organic pollutants on the surface, including electrolyte residues, such as organic solvents (dimethyl carbonate, ethylene carbonate, etc.) and lithium salts (LiPF6). These organic substances will adhere to the surface of graphite, affecting its conductivity and may react with graphite to form an unstable interface; (2) Oxidation and cracking of graphite. Oxidized graphite (such as the formation of graphene oxide, carboxyl, aldehyde and other functional groups) will reduce its conductivity and stability. During the charge and discharge process of the battery, the graphite negative electrode material may experience mechanical stress, resulting in the destruction of the crystal structure of the graphite material and cracks on the surface of the particles. These damages will affect the conductivity and overall performance of the graphite; (3) Disordered and amorphous regions. During the battery cycle, the layered structure of graphite may be damaged, resulting in disordered regions (such as amorphous carbon regions and regions with a low degree of graphitization). These defects and disordered regions will affect the overall conductivity of graphite and the ability of lithium ion intercalation / deintercalation.

[0027] The present application first cleans and decontaminates the retired battery graphite material to remove organic matter, metal impurities and electrolyte residues. The pretreated graphite material is mainly composed of a pure graphite matrix, but the surface also contains a small amount of oxides, hydroxyl compounds, removed organic residues, metal impurities and ionic liquid components.

[0028] The pre-treated graphite material is soaked in ionic liquids, which can react with the graphite surface in various ways through chemical reactions or physical effects, including oxidation, reduction, removal of organic impurities, removal of metal impurities, adsorption, and surface functionalization. These reactions help remove impurities on the graphite surface, improve the electrochemical properties of graphite, optimize its surface structure, and improve conductivity and cycle stability, thereby enhancing the performance of the graphite negative electrode in the battery.

[0029] The graphite material soaked in ionic liquid is pyrolyzed to further remove organic pollutants in the graphite material, while restoring or optimizing the structure of the graphite material. In this process, the auxiliary effect of ionic liquid can reduce the pyrolysis temperature, improve the pyrolysis efficiency, and reduce the damage of high temperature environment to graphite.

[0030] After pyrolysis treatment, the organic pollutants in the graphite material are removed, and the structure of the graphite material is restored or optimized, which is manifested in the following changes: (1) Removal of organic matter: The pyrolysis process can effectively decompose and remove organic pollutants on the graphite surface (such as electrolyte residues, polymer binder, solvents, etc.). These substances decompose into gases (such as CO, CO2, HF, etc.) and volatile organic compounds during the pyrolysis process, making the graphite surface cleaner.

[0031] (2) Removal of oxides: During the pyrolysis process, oxides on the graphite surface (such as carboxyl, phenolic hydroxyl, etc.) can also be removed to restore the original state of graphite. Removal of oxides helps to improve the conductivity and electrochemical stability of graphite.

[0032] (3) Restoration and optimization of graphite layer: Restoration of layered structure: Pyrolysis treatment helps to restore the layered structure of graphite, repair the crystal structure of graphite, reduce the presence of amorphous areas, and improve the conductivity of graphite and the cycle stability of the battery. By removing surface impurities and optimizing the interlayer spacing, the structure of graphite becomes more regular, which is conducive to the insertion and extraction of lithium ions, thereby increasing the specific capacity of the battery. Improvement of graphitization degree: At a certain temperature (generally 500°C to 700°C), pyrolysis helps to improve the graphitization degree of graphite, making the graphite crystals larger and the interlayer spacing smaller, thereby further improving the conductivity and electrochemical properties of graphite. The increase in the degree of graphitization means that the amorphous area in the graphite is reduced and the crystal structure is more complete and stable.

[0033] (4) Improvement of surface properties: Surface functionalization: Although most organic pollutants are removed during the pyrolysis process, a small amount of functional groups (such as hydroxyl, carboxyl, graphene oxide, etc.) may sometimes be generated on the graphite surface. These functional groups can provide graphite with better compatibility with the electrolyte, optimize its electrochemical performance, and improve the cycle stability of graphite. Removal of impurities (such as metal ions): The pyrolysis process helps to remove metal impurities (such as lithium, cobalt, nickel and other metal ions) that may remain in the graphite, thereby reducing their negative impact on the electrochemical properties of graphite.

[0034] After recovery through pyrolysis, the organic pollutants and oxides of graphite are removed, the layered structure is restored or optimized, and the conductivity, electrochemical properties and cycle stability of graphite are significantly improved.

[0035] Preferably, the imidazole ionic liquid includes one or more of 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]), 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][PF4]), 1-ethyl-3-methylimidazolium bistrifluoromethanesulfonyl imide salt ([emin]TF2N), and 1-octyl-3-methylimidazolium tetrafluoroborate ([omin][BF4]); the pyridine ionic liquid is one or two of 1-ethyl-1-methylpyrrolidine tetrafluoroborate and N-butyl-N-methylpyrrolidine trifluoromethanesulfonate; and the ammonium salt ionic liquid includes tetrabutylammonium tetrafluoroborate ([N4444][BF4]).

[0036] Preferably, the fluorine-containing ionic liquid includes one or more of 1-ethyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methylimidazolium hexafluorophosphate, and tetramethylammonium hexafluorophosphate.

[0037] In the technical solution of the embodiment of the present application, the ionic liquid undergoes various types of reactions with the graphite surface, mainly including the following reactions: (1) Oxidation reaction: Certain oxygen-containing anions in ionic liquids (such as [PF6], [Tf2N], etc.) may react with the graphite surface to form an oxide layer (such as graphite oxide, hydroxyl, carboxyl, etc.) on the graphite surface. These oxidation products will increase surface functional groups and provide active sites for lithium ion embedding, thereby optimizing the electrochemical properties of graphite.

[0038] (2) Reduction reaction: Some ionic liquids, especially those containing weakly reducing cations (such as [Nnmpy], [Bmim], etc.), may reduce some oxide layers on the graphite surface and restore them to a lower oxidation state. In this way, unnecessary oxides can be removed or the conductivity of the surface can be improved.

[0039] (3) Dissolving polymer binder: When polymer binder (such as PVDF) is exposed to ionic liquid (such as [C4mim][PF6]), PVDF will undergo a dissolution reaction with the ionic liquid to generate soluble substances. The polarity and solubility properties of the ionic liquid help remove organic residues attached to the graphite surface and reduce their negative impact on battery performance.

[0040] (4) Reaction with metal ions: Certain anions in ionic liquids can form soluble complexes with metal ions (such as Cu, Ni, Co, etc. from batteries) to remove metal ions from the graphite surface. For example, [PF6] can form soluble complexes with metal ions (such as lithium ions) to remove them from the graphite surface.

[0041] Preferably, component A is an imidazole ionic liquid, component B is tetramethylammonium hexafluorophosphate, and the volume ratio of component A to component B is (1-3):1.

[0042] In the technical solution of the embodiment of the present application, the addition of component B increases the proportion of fluoride ions in the composite ionic liquid, which can enhance the ability of the ionic liquid to dissolve organic pollutants and metal impurities, and especially has a good removal effect on residual fluoride or metal ions in the battery. Fluoride ions can also help improve the structural stability of graphite, especially the pyrolysis efficiency at high temperatures.

[0043] Imidazole ionic liquids: Imidazole cations have strong surface activity, which can effectively improve the surface structure of graphite, optimize the electrochemical properties of graphite materials, and help remove surface organic pollutants. Too low a ratio may result in insufficient surface functionalization, while too high a ratio may increase the viscosity of the ionic liquid and affect the treatment efficiency.

[0044] In this embodiment, component A is a pyridine ionic liquid, component B is a fluorine-containing ionic liquid, and the volume ratio of component A to component B is (1-2):1.

[0045] In the technical solution of the embodiment of the present application, the addition of component B increases the proportion of fluoride ions in the composite ionic liquid, which can improve the wettability and conductivity of the graphite surface, and at the same time has a strong decontamination ability, especially in the removal of metal ions and organic impurities. This type of combination is usually used in situations where highly optimized electrochemical performance is required, especially for applications where the cycle stability and conductivity of graphite need to be improved.

[0046] Pyridine cations: Pyridine cations can provide a certain catalytic effect and help functionalize the surface of graphite materials, but too low a ratio may affect the overall de-doping efficiency. Pyridine ions are usually used to enhance the compatibility between graphite and electrolyte and improve ionic conductivity.

[0047] In this embodiment, component A is an ammonium salt ionic liquid, component B is one or both of 1-ethyl-3-methylimidazolium hexafluorophosphate and 1-ethyl-3-methylimidazolium hexafluorophosphate, and the volume ratio of component A to component B is (1-5):1.

[0048] In the technical solution of the embodiment of the present application, ammonium salt cations: ammonium salt cations (such as [N4444]) generally have strong affinity and can effectively remove moisture and other inorganic impurities from the graphite surface, but their solubility is low, so they generally require an appropriate proportion to ensure their effect.

[0049] Fluoride anions: Fluoride anions (such as [PF6], [BF4]) have strong electrochemical stability and conductivity and are suitable for situations where graphite materials need to be processed at high temperatures. Their main function is to help improve the conductivity of graphite and reduce the pyrolysis temperature during the processing process.

[0050] Preferably, in step S2, the mass ratio of the ionic liquid to the pretreated graphite material during immersion is 1:(1-3).

[0051] Preferably, in step S3, the pyrolysis temperature is 300-600° C., and the pyrolysis time is 0.5-3 h.

[0052] In the technical solution of the embodiment of the present application, the auxiliary effect of the ionic liquid can reduce the pyrolysis temperature, improve the pyrolysis efficiency, and reduce the damage of high temperature to graphite.

[0053] (1) High thermal stability of ionic liquids: Ionic liquids generally have high thermal stability, which means that they do not decompose easily at high temperatures. Therefore, they can exist stably for a long time during the pyrolysis of graphite, promoting the degradation or removal of organic matter without causing the decomposition of the ionic liquids themselves. Ionic liquids reduce the temperature required for pyrolysis by interacting with impurities.

[0054] (2) Dissolving power and catalytic effect of ionic liquids: Ionic liquids have unique dissolving power and catalytic properties. During the pyrolysis process, the cations and anions in the ionic liquids can react strongly with organic pollutants on the graphite surface, polymer binders (such as PVDF), and electrolyte residues, promoting the degradation of these substances; ionic liquids accelerate the degradation process of organic pollutants through catalysis, thereby reducing the temperature required for pyrolysis.

[0055] (3) Viscosity and thermal conductivity of ionic liquids: The high viscosity and low thermal conductivity of ionic liquids are also one of the reasons why they can reduce the temperature during the pyrolysis process. Since ionic liquids can form a protective film on the surface, they limit the rapid spread of heat, thereby reducing the pyrolysis temperature of graphite.

[0056] Not all ionic liquids can effectively reduce the pyrolysis temperature. The specific effect of ionic liquids depends on the chemical properties of their cations and anions, solubility, thermal stability, and interaction with impurities. Therefore, the characteristics and selection of ionic liquids are crucial, and appropriate ionic liquids must be selected based on the type of target impurities and the pyrolysis temperature requirements.

[0057] Preferably, in step S1, the graphite material is cleaned using one or more of deionized water, an organic solvent or an acidic solution.

[0058] Preferably, in step S4, the pyrolyzed graphite material is washed alternately with deionized water and ethanol for 2 to 3 times, and then dried at a temperature of 100 to 120° C. for 6 to 8 hours.

[0059] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. The reagents or instruments used without specifying the manufacturer are all conventional products that can be obtained commercially.

[0060] 1. Preparation method Example 1 (1) Raw materials and preparation Retired batteries: Use graphite negative electrode materials recovered from scrapped lithium-ion batteries, mechanically crush the graphite negative electrode materials, and use clean water and acetone to ultrasonically clean the graphite negative electrode materials in turn. After cleaning, heat and dry to remove external impurities, and control the particle size to 50~100 μm.

[0061] Ionic liquid: A mixture of 1-butyl-3-methylimidazolium hexafluorophosphate and tetramethylammonium hexafluorophosphate was used in a ratio of 1:1.

[0062] (2) Ionic liquid immersion treatment: Mix the retired battery graphite material with the ionic liquid for 12 hours at a temperature of 80°C. The organic pollutants, residual electrolyte, polymer binder and metal impurities on the graphite surface are removed by the dissolution effect of the ionic liquid.

[0063] (3) Pyrolysis treatment: The graphite material pretreated with ionic liquid is transferred to a high-temperature furnace for pyrolysis treatment. The pyrolysis temperature is set at 500°C and maintained at this temperature for 2 hours. The treatment is carried out under a nitrogen atmosphere to avoid oxidation reactions. This process aims to remove residual organic pollutants, restore the structure of graphite, and reduce oxides and amorphous areas.

[0064] Example 2 The difference between Example 2 and Example 1 is that the ionic liquid is: 1-ethyl-1-methylpyrrolidine tetrafluoroborate and 1-ethyl-3-methylimidazolium hexafluorophosphate are mixed in a volume ratio of 2:1, and the other steps are the same as Example 1.

[0065] Example 3 The difference between Example 3 and Example 1 is that the ionic liquid is a mixture of tetrabutylammonium tetrafluoroborate and 1-ethyl-3-methylimidazolium hexafluorophosphate in a volume ratio of 2:1, and the other steps are the same as Example 1.

[0066] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the ionic liquid immersion treatment is not used, and the pyrolysis treatment is directly used. The other steps are the same as those in Example 1.

[0067] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that after the ionic liquid immersion treatment, no pyrolysis treatment is used, and the other steps are the same as those in Example 1.

[0068] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the ionic liquid is 1-ethyl-3-methylimidazolium hexafluorophosphate, and the other steps are the same as those of Example 1.

[0069] 2. Test Method 1. Graphite purity test method: Use X-ray photoelectron spectroscopy (XPS) to analyze the elemental composition of graphite surface, and Fourier transform infrared spectroscopy (FTIR) to detect the residue of organic matter.

[0070] 2. Graphite regeneration efficiency test method: Analyze the structural changes of graphite through specific surface area test (using BET specific surface area meter) and X-ray diffraction (XRD) to evaluate the regeneration efficiency.

[0071] 3. Electrochemical performance test method: Use constant current charge and discharge test (in common battery test systems, the cycle current density is 0.1C) and cyclic voltammetry (CV) to test the electrochemical performance of graphite.

[0072] III. Analysis of test results of various embodiments and comparative examples The oxide content, specific surface area, regeneration efficiency and capacity decay rate of the retired battery graphite before treatment and each embodiment and comparative example were tested. The test results are shown in Table 1 below.

[0073] Table 1 Retired battery graphite and test results of various embodiments and comparative examples

[0074] The surface of the retired battery graphite before treatment has a high oxygen content (oxides such as carboxyl, phenolic hydroxyl, etc.) and a large amount of organic pollutants (such as carbonates and polymer residues); the specific surface area of ​​the retired battery graphite before treatment is relatively low, at 10m 2 / g, with a certain degree of disordered structure, large interlayer spacing and more crystal defects; the initial capacity of retired battery graphite before treatment was 150mAh / g, but with the charge and discharge cycle, its capacity decayed rapidly, and the capacity after 100 cycles dropped to 70mAh / g, with a capacity decay rate of 53.3%.

[0075] The graphite regenerated by the method in Example 1 has a significantly reduced oxygen content, completely removed organic pollutants, improved graphite purity, reduced oxide content to <1%, and almost no organic matter remains; after pyrolysis, the specific surface area of ​​the graphite increases to 25m 2 / g, the interlayer spacing recovered to a lower value (about 0.335 nm), the degree of graphitization was improved, the amorphous area was significantly reduced, and the regeneration efficiency exceeded 90%; the initial capacity of the graphite after ionic liquid-assisted pyrolysis treatment was restored to 220 mAh / g, and after 500 charge and discharge cycles, it can still maintain a capacity of 180 mAh / g, showing high stability and low capacity decay rate (decay rate is about 18%).

[0076] In Comparative Example 1, no ionic liquid was used, and the graphite material was only treated by high-temperature pyrolysis. The results showed that the purity of the treated graphite was low, the oxide content was high, the electrochemical performance was poor, and the capacity recovery was not obvious.

[0077] In Comparative Example 2, no pyrolysis was performed, and only ionic liquid was used to treat the graphite. The results showed that although the organic pollutants were removed, the structure of the graphite was not significantly optimized and the electrochemical performance was limited.

[0078] In Comparative Example 3, only fluorine-containing ionic liquid was used for immersion, and the regeneration effect was lower than that of Examples 1 to 3, indicating that a better regeneration effect of graphite material can be achieved by combining multiple ionic liquids.

[0079] The test data of the above-mentioned embodiments and comparative examples prove that the ionic liquid-assisted pyrolysis method can effectively restore the structure of retired battery graphite negative electrode materials, significantly improve their purity, electrochemical properties and regeneration efficiency, which provides an efficient and feasible treatment solution for the recovery and reuse of retired battery graphite materials.

[0080] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. An ionic liquid-assisted pyrolysis method for regenerating retired battery graphite material, characterized in that: The steps include: S1. Cleaning and removing impurities from retired battery graphite materials to obtain pretreated graphite materials; S2, soaking the pretreated graphite material with a composite ionic liquid, wherein the composite ionic liquid comprises component A and component B, wherein component A and component B are mutually soluble, wherein component A comprises any one of an imidazole ionic liquid, a pyridine ionic liquid and an ammonium salt ionic liquid, and component B is a fluorine-containing ionic liquid; S3, pyrolyzing the soaked graphite material under an inert atmosphere; S4, washing and drying the pyrolyzed graphite material to obtain regenerated graphite.

2. The ionic liquid-assisted pyrolysis method for regenerating retired battery graphite material according to claim 1, characterized in that: The imidazole ionic liquid includes one or more of 1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium tetrafluoroboric acid, 1-ethyl-3-methylimidazolium bistrifluoromethanesulfonyl imide salt, and 1-octyl-3-methylimidazolium tetrafluoroborate; the pyridine ionic liquid is one or two of 1-ethyl-1-methylpyrrolidine tetrafluoroborate and N-butyl-N-methylpyrrolidine trifluoromethanesulfonate; and the ammonium salt ionic liquid includes tetrabutylammonium tetrafluoroborate.

3. The ionic liquid-assisted pyrolysis method for regenerating retired battery graphite material according to claim 1, characterized in that: The fluorine-containing ionic liquid includes one or more of 1-ethyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methylimidazolium hexafluorophosphate, and tetramethylammonium hexafluorophosphate.

4. The ionic liquid-assisted pyrolysis method for regenerating retired battery graphite material according to claim 1, characterized in that: The component A is an imidazole ionic liquid, the component B is tetramethylammonium hexafluorophosphate, and the volume ratio of the component A to the component B is (1-3):

1.

5. The ionic liquid-assisted pyrolysis method for regenerating retired battery graphite material according to claim 1, characterized in that: The component A is a pyridine ionic liquid, the component B is a fluorine-containing ionic liquid, and the volume ratio of the component A to the component B is (1-2):

1.

6. The ionic liquid-assisted pyrolysis method for regenerating retired battery graphite material according to claim 1, characterized in that: The component A is an ammonium salt ionic liquid, the component B is one or both of 1-ethyl-3-methylimidazolium hexafluorophosphate and 1-ethyl-3-methylimidazolium hexafluorophosphate, and the volume ratio of the component A to the component B is (1-5):

1.

7. The ionic liquid-assisted pyrolysis method for regenerating retired battery graphite material according to claim 1, characterized in that: In the step S2, the mass ratio of the ionic liquid to the pretreated graphite material during immersion is 1:(1-3).

8. The ionic liquid-assisted pyrolysis method for regenerating retired battery graphite material according to claim 1, characterized in that: In step S3, the pyrolysis temperature is 300-600° C., and the pyrolysis time is 0.5-3 h.

9. The ionic liquid-assisted pyrolysis method for regenerating retired battery graphite material according to claim 1, characterized in that: In the step S1, one or more of deionized water, organic solvent or acidic solution is used to clean the graphite material.

10. The ionic liquid-assisted pyrolysis method for regenerating retired battery graphite material according to claim 1, characterized in that: In step S4, the pyrolyzed graphite material is washed alternately with deionized water and ethanol for 2 to 3 times, and then dried at a temperature of 100 to 120° C. for 6 to 8 hours.

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