Regenerated graphite material, preparation method and application

Abstract

The embodiment of the invention provides a regenerated graphite material as well as a preparation method and application thereof. The preparation method comprises the following steps: mixing a graphite catalyst with waste graphite to obtain a composite precursor; the graphite catalyst is composed of transition metal and liquefiable metal; the melting point of the liquefiable metal is lower than that of the waste graphite; and placing the composite precursor in a preset gas environment, and heating the composite precursor, so that the graphite catalyst is liquefied and permeates into the waste graphite in a flowing manner, the graphite structure of the waste graphite is repaired, and regenerated graphite for manufacturing the battery negative electrode is obtained. A graphite catalyst composed of a transition metal and a low-melting-point liquefiable metal permeates into the waste graphite, and a damaged graphite layer structure of the waste graphite is catalytically repaired. The obtained regenerated graphite material can be used for preparing a battery negative electrode, so that the raw material cost and the resource consumption are reduced, and the environmental burden caused by a waste battery negative electrode material is also reduced.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to the technical field of batteries, in particular to a regenerated graphite material, a preparation method and an application thereof. BACKGROUND

[0002] With the rapid development of new energy vehicles, energy storage systems and consumer electronics, the production and use of lithium ion batteries continue to rise, followed by the recycling and resource utilization of a large number of retired power batteries. Graphite, as the core material of the negative electrode of lithium ion batteries, accounts for about 10%-15% of the total mass of the battery. During the service of the battery, the negative electrode graphite will be structurally damaged due to repeated lithium ion insertion / extraction, such as expansion of interlayer spacing, lattice distortion, formation of irreversible SEI film on the surface, and particle breakage, etc., resulting in significant attenuation of its electrochemical performance, and eventually being scrapped together with the battery.

[0003] Currently, the recycling of waste batteries mainly focuses on the extraction of valuable metals (such as cobalt, nickel and manganese) in the positive electrode material, while the negative electrode graphite is mostly treated by incineration, landfill or simple crushing for use as low-end carbon materials, which not only causes resource waste, but also may cause environmental pollution. SUMMARY

[0004] In view of the above-mentioned shortcomings of the prior art, the purpose of the present disclosure is to provide a regenerated graphite material, a preparation method and an application thereof, to solve the problems in the related art.

[0005] The first aspect of the present disclosure provides a preparation method of a regenerated graphite material, which is applied to a battery negative electrode, comprising:

[0006] mixing the graphite catalyst with waste graphite to obtain a composite precursor; the graphite catalyst is composed of a transition metal and a liquefiable metal; the melting point of the liquefiable metal is lower than the melting point of the waste graphite;

[0007] placing the composite precursor in a predetermined gas environment, and heating the composite precursor to liquefy and flow the graphite catalyst into the waste graphite, so as to repair the graphite structure of the waste graphite, and obtain a regenerated graphite for manufacturing the battery negative electrode.

[0008] In an embodiment of the first aspect, the transition metal at least includes one or more of Ni, Co and Fe; and / or,

[0009] The liquefiable metal at least includes Ga, In, Sn and corresponding liquid eutectic alloys thereof.

[0010] In an embodiment of the first aspect, the preparation method of the graphite catalyst comprises:

[0011] mixing the powder of the transition metal and the liquefiable metal in a preset mass ratio to obtain a first mixture;

[0012] grinding the first mixture to allow the liquefiable metal to coat the surface of the transition metal to form the graphite catalyst;

[0013] alternatively,

[0014] dispersing the liquefiable metal with a polar solvent and removing the oxide layer of the liquefiable metal with an acidic solvent to obtain a liquefiable metal to be reacted;

[0015] adding the transition metal in a preset proportion to the liquefiable metal to be reacted and performing ultrasonic treatment in an inert atmosphere or a reducing atmosphere to allow the transition metal to combine with the liquefiable metal to be reacted, thereby obtaining the graphite catalyst.

[0016] In an embodiment of the first aspect, the mixing of the graphite catalyst and the waste graphite to obtain a composite precursor uses dry ball milling or wet ultrasonic treatment to allow the graphite catalyst to infiltrate the surface of the waste graphite to obtain the composite precursor.

[0017] In an embodiment of the first aspect, the composite precursor is subjected to a heating treatment in a sealed high-temperature reaction container.

[0018] In an embodiment of the first aspect, the preset gas environment includes an inert gas or a reducing gas; and the heating treatment includes heating at a rate of 2–5°C·min⁻¹ to 500–900°C and maintaining the temperature for 1–3 hours.

[0019] In an embodiment of the first aspect, the regenerated graphite is made into the negative electrode of the battery using a water-soluble binder.

[0020] In an embodiment of the first aspect, the repairing of the graphite structure of the waste graphite further includes separating the regenerated graphite and the transition metal and the liquefiable metal used to prepare the graphite catalyst by at least one of the following methods: alkali dissolution and acid precipitation, acid leaching and solvent extraction, and physical separation.

[0021] The second aspect of the present disclosure provides a regenerated graphite material, wherein the regenerated graphite material is prepared by any one of the embodiments of the first aspect.

[0022] The third aspect of the present disclosure provides a use of the regenerated graphite material of the second aspect, wherein the regenerated graphite material is used as a negative electrode material of a battery.

[0023] The beneficial effects of the present disclosure: through the graphite catalyst composed of transition metal and low-melting point liquefiable metal, permeating into the waste graphite, catalyzing the damaged graphite layer structure of waste graphite. The obtained regenerated graphite material can be used to prepare battery negative electrode, not only reducing the raw material cost and resource consumption, but also reducing the environmental burden brought by waste battery negative electrode material. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The overall flowchart of the preparation method of the regenerated graphite in an embodiment of the present disclosure is shown.

[0025] Figure 2 The flowchart of the preparation method of the graphite catalyst in the preparation method in an embodiment of the present disclosure is shown.

[0026] Figure 3 The flowchart of the preparation method of the graphite catalyst in the preparation method in another embodiment of the present disclosure is shown.

[0027] Figure 4 The XRD graph of the regenerated graphite in an embodiment of the present disclosure is shown.

[0028] Figure 5 The SEM graph of the regenerated graphite in an embodiment of the present disclosure is shown.

[0029] Figure 6 The first circle charge-discharge curve of the regenerated graphite in an embodiment of the present disclosure at 0.1C is shown.

[0030] Figure 7 The long cycle curve of the regenerated graphite in an embodiment of the present disclosure at 1.0C is shown. DETAILED DESCRIPTION

[0031] The embodiments of the present disclosure are described below through specific specific examples, and those skilled in the art can easily understand other advantages and effects of the present disclosure from the disclosed information. The present disclosure can also be implemented or applied by other different specific embodiments, and the details in the present disclosure can be modified or changed according to different views and applications without departing from the spirit of the present disclosure. It should be noted that the embodiments in the present disclosure and the features in the embodiments can be combined with each other without conflict.

[0032] The embodiments of the present disclosure are described in detail below with reference to the accompanying drawings, so that those skilled in the art can easily implement the present disclosure. The present disclosure can be embodied in various different forms, and is not limited to the embodiments described here.

[0033] In the description of the present disclosure, the expressions "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" etc. mean that the particular feature, structure, material or characteristic following the expressions are included in at least one embodiment or example of the present disclosure. Also, the expressions can include a particular feature, structure, material or characteristic in combination with one or more of the other features, structures, materials or characteristics in any one or more embodiments or examples. In addition, the different embodiments or examples of the present disclosure and the features of the different embodiments or examples can be combined and combined with each other, if not mutually exclusive.

[0034] In addition, the terms "first", "second", etc. are used only to indicate a purpose, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second" can explicitly or implicitly include at least one of the features. In the description of the present disclosure, the meaning of "a group" is two or more, unless specifically limited.

[0035] In order to clearly illustrate the present disclosure, devices irrelevant to the description are omitted, and the same reference numerals are assigned to the same or similar constituent elements throughout the description.

[0036] Throughout the description, when it is said that a device is "connected" to another device, it includes not only the case of "direct connection", but also the case of "indirect connection" in which other elements are placed therebetween. In addition, when it is said that a device "includes" a certain constituent element, unless specifically stated to the contrary, other constituent elements are not excluded, but it means that other constituent elements can also be included.

[0037] Although the terms first, second, etc. are used herein to refer to various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first interface and a second interface, etc. are indicated. Furthermore, as used herein, the singular forms "a", "an" and "the" are intended to include plural forms, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises", "comprising", "includes" and / or "including", mean the presence of the stated features, steps, operations, elements, modules, items, kinds and / or groups, but do not exclude the presence or addition of one or more other features, steps, operations, elements, modules, items, kinds and / or groups. The terms "or" and "and / or" as used herein are to be interpreted as inclusive, or meaning either or any combination. Therefore, "A, B or C" or "A, B and / or C" means "any of the following: A; B; C; A and B; A and C; B and C; A, B and C". This definition applies only when a combination of elements, functions, steps or operations are in some way specifically called out in a claim.

[0038] The professional terms used herein are only used to refer to specific embodiments and are not intended to limit the disclosure. The singular form used herein, unless the statement explicitly indicates the opposite meaning, also includes the plural form. The meaning of "include" used in the specification is to specify the specific features, regions, integers, steps, operations, elements and / or components, and is not to exclude the presence or addition of other features, regions, integers, steps, operations, elements and / or components.

[0039] Although not defined differently, the technical terms and scientific terms used herein include the meanings commonly understood by those skilled in the art to which the present disclosure belongs. The terms defined in the commonly used dictionary are additionally explained to have meanings consistent with the relevant technical literature and the currently prompted message, unless defined, and should not be interpreted as ideal or very formal meanings.

[0040] Graphite, as the core negative material of energy storage devices such as lithium ion batteries, sodium ion batteries, etc., has long dominated the commercial battery negative market due to its advantages such as layered crystal structure (favorable for lithium ion / sodium ion intercalation and deintercalation), high electronic conductivity (low electrode impedance), excellent cycle stability and controllable cost. However, with the rapid development of the energy storage industry, the number of waste graphite is increasing, so graphite regeneration has urgent technical and industrial needs, which can be developed in the following three aspects:

[0041] The preparation of graphite (especially high-purity battery-grade graphite) depends on the mining of natural graphite ore or artificial synthesis. The former is limited by the non-renewable nature of mineral resources, and the latter requires high-temperature graphitization (traditional process temperature exceeds 2000℃), which has high energy consumption and cost. After a long cycle (usually 500-1000 times) or retirement, the graphite negative electrode is not completely disabled, and the performance is degraded due to interlayer peeling (abnormal expansion of interlayer spacing caused by repeated intercalation of lithium ions), surface contamination (attachment of electrolyte decomposition products), and destruction of the conductive network (breakage of inter-particle connections). If such waste graphite is discarded directly, not only will it cause waste of valuable carbon resources, but also will exacerbate the supply pressure of graphite raw materials.

[0042] In order to realize the reuse of waste graphite, an embodiment of the present disclosure provides a preparation method of regenerated graphite material.

[0043] Figure 1 The overall flowchart of the preparation method of the regenerated graphite in an embodiment of the present disclosure is shown.

[0044] In Figure 1 In an embodiment, step S100: mixing the graphite catalyst with waste graphite to obtain a composite precursor; the graphite catalyst is composed of a transition metal and a liquefiable metal; the melting point of the liquefiable metal is lower than the melting point of the waste graphite.

[0045] The semi-liquid graphite catalyst is formed by mixing and heating a transition metal and a liquefiable metal, wherein the transition metal cannot be completely liquefied and thus exists in a solid state in the graphite catalyst, and a solid-liquid contact mode is used to replace a conventional solid-solid contact, and the liquefiable metal is used to realize sufficient penetration of the catalyst into a defect region of the graphite.

[0046] The transition metal is an element in a d block (groups 3-12) of a periodic table (part of elements in the d block are included), and a valence electron layer of the transition metal contains uncompleted d orbital electrons. The transition metal is typically iron (Fe), cobalt (Co), or nickel (Ni), has strong catalytic activity, can reduce a reaction activation energy, promotes graphitization, and has variable valence to improve catalytic efficiency and structural stability, and is a core active component for repairing the graphite.

[0047] Optionally, in an embodiment of the present application, the transition metal includes at least one or more of Ni, Co, and Fe, and a form of the transition metal is a simple metal particle or a soluble metal salt. The metal salt is selected from any one or more combinations of chlorides, nitrates, sulfates, acetates, hydrated salts, or coordination complexes of corresponding elements. When the simple form is used, the transition metal can directly play a catalytic role in a heating process. When the soluble salt form is used, the transition metal can be uniformly dispersed at a molecular level with the waste graphite powder and the liquefiable metal in a mixing stage, and then is reduced to an active metal phase in situ in a heat treatment process, so that the distribution uniformity and the catalytic efficiency of the catalyst in the graphite structure repairing process are effectively improved.

[0048] The liquefiable metal is implemented as a liquid metal (LM) that is a metal or a metal alloy in a liquid state under specific temperature conditions, has flowability, wettability, and controllability, can penetrate into a small pore and closely adhere to a graphite surface.

[0049] Optionally, the liquefiable metal includes at least simple substances of Ga, In, and Sn and corresponding liquid eutectic alloys thereof.

[0050] Specifically, the melting point of gallium (Ga) is about 29.8°C, the melting point of indium (In) is about 156.6°C, and the melting point of tin (Sn) is about 231.9°C, which is still significantly lower than a high temperature required for structural reconstruction of the graphite, usually 2500°C or higher.

[0051] Further, when the liquefiable metal forms a eutectic alloy at a specific ratio, the melting point of the liquefiable metal can be further greatly reduced, and even a liquid state at room temperature can be achieved, for example:

[0052] EGaIn (Eutectic Gallium-Indium) has a melting point of approximately 15.7°C, is a stable liquid at room temperature, and exhibits excellent electrical conductivity and wettability.

[0053] EGaSn (Eutectic Gallium-Tin) has a melting point that can be lowered to about 20°C.

[0054] Galinstan has a melting point range of approximately −19°C to +10°C, is completely liquid at room temperature, has better chemical stability than mercury, and exhibits good wetting ability on carbon materials.

[0055] In this application, the aforementioned liquefiable metal or its eutectic alloy serves as the "liquefiable metal" component, which, together with transition metals (such as Ni, Co, Fe, etc.), constitutes the graphite catalyst. During the heat treatment stage, typically within the range of 500–1500°C, the liquefiable metal melts first and forms a continuous liquid phase. This not only effectively dissolves or disperses the active transition metal component but also, due to its high surface tension and fluidity, penetrates into the microcracks, interlayer defects, and surface pores of the waste graphite powder particles.

[0056] Liquefiable metals serve as both "transport carriers" and "contact media," using their fluidity to deliver transition metals to deep defects in graphite. This "solid-liquid contact" replaces the limitations of traditional transition metal "solid-solid contact," allowing the catalytic activity of the transition metals to fully engage with the disordered graphite structure. This enables layered reconstruction and repair of the conductive network, and the metals can be subsequently recycled through separation processes. This approach not only solves the problems of low catalytic efficiency and heavy pollution associated with traditional processes but also avoids resource waste.

[0057] Optionally, the graphite catalyst can be prepared by a powder grinding solid-phase method and a salt-ultrasonic liquid-phase method. Both methods are suitable for different raw material forms of transition metals, such as elemental powders and soluble salts, and can ultimately form graphite catalysts with graphite remediation catalytic activity through the reaction of transition metals and liquefiable metals.

[0058] Furthermore, this application does not limit the preparation method of graphite catalysts to a single method. Besides the two methods described below, other preparation methods that can effectively combine transition metals and liquefiable metals and produce a final product with catalytic graphitization capability are all within the scope of protection of this application. The two preparation methods are described in detail below:

[0059] Figure 2 The example uses a powder grinding solid-phase method to obtain a graphite catalyst. Through the physical action of mechanical grinding, the liquefiable metal is dispersed and coated on the surface of the solid transition metal powder, which has the advantages of simple process operation and no solvent residue.

[0060] exist Figure 2 The embodiments specifically include:

[0061] Step A110: mixing the powder of transition metal and the liquefiable metal in a preset mass ratio to obtain a first mixture.

[0062] The powder of transition metal is selected from one or more of Ni, Co, and Fe, and has a particle size ranging from 50 nm to 10 µm. The powder can be a monodisperse pure metal powder, such as a Ni powder with a particle size of 100 nm, or a multi-metal mixed powder, such as a composite powder with a mass ratio of Ni to Co of 1:1.

[0063] The liquefiable metal can be selected from Ga, In, and Sn, or a low-melting-point eutectic alloy thereof, such as EGaIn, Galinstan, and EGaSn. It is confirmed in advance that the liquefiable metal is in a liquid state at an operating temperature, such as room temperature or the temperature in a glove box. If the ambient temperature is lower than the melting point of the liquefiable metal, the liquefiable metal can be melted by low-temperature heating (such as 30-50°C) before use.

[0064] The inert atmosphere includes N2 and / or Ar, which isolates O2 and H2O in the air, prevents the powder of transition metal from being oxidized to form an oxide that has no catalytic activity, and avoids the formation of a dense oxide layer on the surface of the liquefiable metal, which affects the coating effect.

[0065] The raw materials are weighed according to a preset mass ratio, such as a mass ratio of transition metal to liquefiable metal of 1:10-1:1, and are placed in a ball mill jar or an agate / ceramic mortar. The mass ratio needs to be adjusted according to the actual catalytic requirements. If it is necessary to enhance the catalytic activity, such as repairing high-defect graphite, the proportion of transition metal can be increased. If it is necessary to improve the fluidity of the liquefiable metal, the proportion of transition metal can be reduced.

[0066] Step A120: grinding the first mixture to allow the liquefiable metal to wrap the surface of the transition metal to form the graphite catalyst.

[0067] Further, the graphite catalyst is obtained by any one of the following grinding methods: manual mortar grinding and planetary ball milling. Specifically:

[0068] Manual mortar grinding: an agate or ceramic mortar (which is used to avoid metal contamination) is used for circumferential grinding at a uniform force for 30-45 minutes. The inner wall of the mortar is scraped once every 10 minutes to ensure that the raw materials are fully mixed and that there is no residue. Finally, the graphite catalyst is obtained.

[0069] Planetary ball milling: the rotation speed is set to 300-500 rpm, the ball-to-material ratio is 10±2 (the grinding balls are selected from agate balls or 316L stainless steel balls to avoid chemical reactions with the raw materials), and the ball milling time is 30-90 minutes. Further, to prevent the liquefiable metal from volatilizing due to overheating of the system, an intermittent program of “stopping for 5 minutes every 30 minutes” can be set to ensure the stability of the grinding process.

[0070] During the grinding process, the liquefiable metal is gradually broken into micro-nano level droplets (particle size 1-10 μm) under the action of mechanical force (grinding friction, ball milling impact force), and at the same time, it is coated on the surface of the transition metal powder by surface tension, forming a graphite catalyst. The obtained graphite catalyst does not need subsequent purification treatment and can be directly mixed with waste graphite powder to prepare a composite precursor.

[0071] Figure 3 The embodiment is to obtain a graphite catalyst by a salt-ultrasonic liquid phase method. A transition metal soluble salt is used as a raw material. Through a combination process of "solvent dispersion-acidic oxidation layer removal-ultrasonic assisted reaction", the loading of the transition metal on the surface of the liquefiable metal is realized. The graphite catalyst has the advantages of uniform dispersion of active sites and high catalytic efficiency, and is suitable for the preparation of high-purity regenerated graphite. The specific process is as follows:

[0072] Please refer to Figure 3 The embodiment specifically comprises:

[0073] Step B110: dispersing the liquefiable metal by using a polar solvent and removing the oxidation layer of the liquefiable metal by using an acidic solvent to obtain a liquefiable metal to be reacted.

[0074] Specifically, the polar solution is used to realize the uniform dispersion of the liquefiable metal (such as N, N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), isopropyl alcohol (IPA), ethylene glycol (EG), ethanol (EtOH), etc.), and to construct a stable liquid phase environment for the subsequent reaction. The liquefiable metal is in a liquid state at room temperature, but its surface is prone to "liquid droplet aggregation" due to high interfacial tension. If the liquefiable metal is directly reacted with the transition metal salt, it will lead to insufficient contact and uneven compounding. The polar solution molecules have clear positive and negative centers (such as the amide group in DMF and the sulfoxide group in DMSO), which can be adsorbed on the surface of the liquefiable metal, weaken the attraction between the liquefiable metal droplets, and disperse them into smaller droplets to form a stable liquid phase dispersion system.

[0075] In the salt-ultrasonic liquid phase method, the micro-bubbles generated by ultrasonic will release local energy when they break, and the viscosity and surface tension of the polar solution are more easily matched with the ultrasonic energy. The polar solution can not only further disperse the agglomerates of the liquefiable metal, but also provide a good solubility environment for the transition metal salt (such as Ni²+, Co²+ soluble salt), so that the metal ions are uniformly dispersed in the liquid phase, avoiding the problem of particle agglomeration caused by too high local metal ion concentration.

[0076] The acidic solution is used to remove the oxidation layer on the surface of the liquefiable metal, optimize the reaction activity, and enhance the reduction coating effect of the transition metal salt. For example, hydrochloric acid, the concentration of which is usually controlled at 0.05-0.15 mol・L⁻¹.

[0077] Liquefiable metals (such as Ga and In) are reactive metals that readily react with oxygen in the air, forming a dense oxide film (such as Ga₂O₃ and In₂O₃) on their surface. This oxide film isolates the metal lumen (LM) from the external environment, hindering the adsorption and bonding of transition metals to the LM surface and reducing the LM's fluidity and reactivity. However, acidic solutions (such as hydrochloric acid) can dissolve this oxide layer through acid-base reactions (such as Ga₂Ga₂O₃ + 6HCl = 2GaCl₃ + 3H₂O), exposing the LM's metal surface.

[0078] Acidic solutions can also increase the pH value to enhance the reaction behavior of transition metal salts: on the one hand, an appropriate acidic environment can inhibit the hydrolysis of transition metal ions (such as Fe³+, Co²+) (avoiding the formation of hydroxide precipitates, which would lead to the loss of metal ions), ensuring that they exist stably in the liquid phase in the form of free ions, making them more easily adsorbed by the LM surface.

[0079] Step B120: Add the transition metal in a predetermined proportion to the liquefiable metal to be reacted, and perform ultrasonic treatment in an inert or reducing atmosphere to allow the transition metal to bind to the liquefiable metal to be reacted, thereby obtaining the graphite catalyst.

[0080] Specifically, a soluble metal salt of a transition metal (such as chloride, nitrate, sulfate, acetate, or its hydrate / complex) is added to the mixture of the liquefiable metals to be reacted, comprising 10%–50% of the mass of the liquefiable metals.

[0081] The entire reaction process is carried out under an inert atmosphere (such as N2 or Ar) to prevent metal oxidation. A probe-type ultrasonic instrument is used, continuously treating the metal at a power of 100–750 W in pulse mode for 10–60 minutes. Optionally, each ultrasonic session lasts 5–15 seconds, with an interval of 0.5–2 seconds. During this process, high-intensity ultrasonic cavitation promotes micro-fragmentation, emulsification, and interface renewal of the liquefiable metal, allowing the soluble metal salt to be fully dispersed and adsorbed on the surface of the liquefiable metal, and partially reduced to metal nanoparticles.

[0082] Furthermore, it can be placed in a reducing atmosphere containing 5 vol% H2 (such as 5% H2 / Ar), which can promote the in-situ complete reduction of soluble metal salts, forming transition metal nanoparticles uniformly dispersed in the matrix of the liquefiable metal, thereby obtaining the graphite catalyst.

[0083] Back Figure 1 Optionally, in step S100, the graphite catalyst is impregnated on the surface of the waste graphite by dry ball milling or wet ultrasonication to obtain the composite precursor.

[0084] For example, the implementation method of dry ball milling is described.

[0085] In some embodiments, the graphite catalyst is mixed with waste graphite in a certain proportion, and optionally, 2–10 wt% of a polar solvent is added, wherein the polar solvent may be ethanol, isopropanol, or ethylene glycol. Subsequently, planetary ball milling is performed at 200–400 rpm for 20–40 minutes to ensure that the graphite catalyst fully coats and wets the surface of the waste graphite, thereby obtaining a composite precursor.

[0086] For example, we can illustrate the implementation of wet ultrasound.

[0087] In some embodiments, the graphite catalyst and waste graphite are co-dispersed in ethylene glycol or isopropanol. Optionally, the mass ratio of the graphite catalyst to waste graphite in the dispersion system is 1:1 to 20:1. 10–50 mM of a mercapto-based surfactant, including β-mercaptoethanol, ethyl-3-mercaptopropionate, mercaptobenzoic acid, mercaptoacetic acid, mercaptobutyric acid, and mercaptopropanol, may be added to the mixture of the graphite catalyst and waste graphite. Optionally, 0.05–0.2 wt% of a dispersion stabilizer, including polyvinylpyrrolidone (PVP) and its different molecular weight forms (PVP-K10, PVP-K30, PVP-K90), its vinyl acetate copolymer (PVP / VA), or copovidone, may be added to the mixture to improve the droplet dispersion stability of the graphite catalyst and its wettability on the surface of waste graphite.

[0088] After ultrasonic treatment of the mixture for 10–60 minutes, the resulting mixture is filtered and vacuum dried at 80–120°C for 2–4 hours. Preferably, it can be rapidly calcined at 300°C for 10 minutes to remove residual additives, ultimately obtaining a composite precursor with a uniform structure and graphite catalyst fully wetted on the surface of waste graphite.

[0089] Please refer to Figure 1 In the embodiment, step S200 involves placing the composite precursor in a preset gas environment.

[0090] Specifically, the preset gas environment can be selected as an inert atmosphere (such as nitrogen N2 or argon Ar) or a reducing atmosphere (such as an H2 / Ar mixture containing 5 vol% H2) to prevent graphite from oxidizing at high temperatures and to promote the reduction and catalytic activity of transition metals.

[0091] Further, the composite precursor is subjected to a heating treatment, which is preferably performed in a closed high-temperature reaction container, so as to effectively inhibit the volatilization loss of the low-melting-point liquefiable metal in the temperature range of 500-900°C, and improve the retention rate of the catalyst component and the subsequent recovery feasibility.

[0092] In specific embodiments, after the composite precursor is loaded into a high-temperature reaction container with a gas-tight structure, the temperature is programmed to rise to 500-900°C at a temperature rising rate of 2-5°C·min⁻¹, and is kept at this temperature for 1-3 hours. The closed high-temperature container includes but is not limited to a quartz screw thread crucible, a corundum screw thread crucible, a gland corundum crucible, a stainless steel or nickel-based alloy high-temperature closed kettle, a sintered ceramic gland container, etc.; and the sealing mode can adopt a screw thread locking, a flange pressing or a mechanical buckle structure, so as to ensure good airtightness under high temperature and atmosphere protection conditions.

[0093] The graphite catalyst is liquefied and flows into the waste graphite, so as to repair the graphite structure of the waste graphite, and obtain regenerated graphite for manufacturing the battery negative electrode.

[0094] In the heat treatment process, the liquefiable metal is directly in a liquid state or forms a flowable liquid phase by melting, and carries the transition metal nanoparticles dispersed or generated in situ to penetrate into the interlayer, crack and surface defect area of the waste graphite powder; under the catalysis of the transition metal, the disordered carbon is rearranged, the graphite crystallites are grown, and the interlayer spacing is optimized, so as to effectively repair the graphite crystal structure. The obtained regenerated graphite has high crystallinity, good electrical conductivity and excellent lithium ion intercalation and deintercalation reversibility, and can be directly used for the preparation of high-performance lithium ion battery negative electrodes, significantly reducing the raw material cost and promoting the green recycling of waste batteries.

[0095] Optionally, the preparation method further comprises, after step S200: using a water-soluble binder to make the regenerated graphite into the battery negative electrode.

[0096] Specifically, the obtained regenerated graphite is mixed with a conductive agent (such as carbon black, carbon nanotube, etc.) and a water-soluble binder (such as sodium carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or a combination thereof) in a certain proportion, deionized water is added to prepare a uniform slurry, and then the slurry is coated on a copper foil current collector, and after drying, rolling and cutting, a negative electrode sheet is obtained. Not only is it environmentally friendly and low in cost, but also has good compatibility with the trace amount of metal or oxide possibly remaining on the surface of the regenerated graphite, which helps to improve the adhesion and cycle stability of the electrode sheet. The negative electrode thus prepared can be used to assemble a lithium ion battery, further verifying the practical value and industrialization potential of the regenerated graphite material of the present application.

[0097] Optionally, the preparation method further comprises separating the regenerated graphite and the transition metal for preparing the graphite catalyst from the liquefiable metal after repairing the graphite structure of the waste graphite by at least one of the following methods: alkali dissolution and acid precipitation, acid leaching and solvent extraction, and physical separation.

[0098] 1) Alkali dissolution and acid precipitation

[0099] The repaired metal-graphite composite is added into a sodium hydroxide (NaOH) solution with a concentration of 4-8 mol / L at a liquid-solid mass ratio of 5-15 mL / g, and stirred and treated at 70-100°C for 0.5-2 hours. Under this condition, the liquefiable metal is converted into a soluble sodium salt complex into the liquid phase, while the transition metal M remains as an insoluble solid phase with the graphite. After solid-liquid separation, the filter residue is washed with deionized water to obtain high-purity regenerated graphite; the filtrate is adjusted to a pH of 1-3 by passing CO2 or dropping inorganic acid, so that Ga, In and the like are precipitated in the form of hydroxides; if Sn is contained, it exists in the form of stannate in the alkali solution, and does not precipitate after acidification, and needs to be further recovered by electrochemical reduction or chemical reduction. The transition metal M remaining in the filter residue can be recovered by magnetic separation or dissolution in a dilute acid with a concentration of 0.1-2 mol / L.

[0100] 2) Acid leaching and solvent extraction

[0101] The composite is placed in an inorganic acid (such as HCl, HNO3 or H2SO4) with a concentration of 1-3 mol / L, and stirred and leached at 40-80°C for 30-90 minutes, so that the LM and part of the M are converted into soluble metal ions into the filtrate, and the graphite remains in the solid phase. After solid-liquid separation, the filter residue is washed with water to obtain regenerated graphite. After adjusting the pH of the filtrate to 1-2, an extraction system composed of a phosphate ester extractant (such as D2EHPA, EHEHPA) and a low-polarity organic solvent (such as n-hexane, kerosene or n-heptane) is added, and oscillated at room temperature for 1-10 minutes.

[0102] Due to the high charge density and oxygen affinity of LM, it preferentially forms a hydrophobic complex with the phosphate group into the organic phase, while the transition metal ions mainly remain in the aqueous phase, thereby realizing selective separation. If Sn is contained, it is difficult to be extracted although it is soluble in acid, and remains in the aqueous phase, which can be recovered by precipitation or electrodeposition.

[0103] 3) Physical separation method

[0104] The repaired composite is incubated at 50-80°C to melt the LM into a flowable state; low-intensity ultrasound or shear stirring is used to promote the detachment of LM from the surface of the graphite to form microdroplets. Subsequently, hot-state centrifugation (2000-5000 rpm, 2-10 minutes) is performed immediately to make the molten LM settle at the bottom by taking advantage of the density difference, and the graphite and part of the M are distributed in the upper layer.

[0105] After the upper layer solid phase is mechanically removed, the transition metal can be further separated by magnetic separation or dilute acid (0.1-2 mol·L⁻¹) treatment, and finally high-purity regenerated graphite is obtained; the bottom LM can be directly recycled for catalyst preparation.

[0106] Further, the separated metal components can be recycled for the preparation of graphite catalysts after reduction treatment:

[0107] The hydroxides or chlorides of Ga / In can be thermally reduced in a H2 atmosphere at 600-900°C, or regenerated into elemental metal by Zn / Al powder metal thermal reduction, electrolysis;

[0108] Sn can be recovered as metallic tin from solution by sodium borohydride, hypophosphorous acid, H2 reduction or electrodeposition;

[0109] The oxides, hydroxides or salts of transition metals M (such as Ni, Co, Fe) can be reduced to metal powder by 500-800°C H2 reduction, carbothermal reduction or electrochemical method.

[0110] The technical solutions of the present disclosure are further described in detail in combination with specific embodiments, which are only used to illustrate the present disclosure and not to limit the scope of the present disclosure.

[0111] Example One

[0112] Step One: Preparation of Semi-liquid Graphite Catalyst

[0113] Ni powder with a particle size of 1-5 µm and Ga liquefiable metal were weighed in a mass ratio of 1:10, transferred to an agate mortar in an argon glove box, and ground for 30 minutes to form a graphite catalyst with good surface coating.

[0114] Step Two: Preparation of Graphite Catalyst-Graphite Composite Precursor

[0115] The prepared Ni-Ga graphite catalyst was mixed with waste graphite powder in a mass ratio of 15:85, 5wt% ethanol was added, and the mixture was ball milled in a planetary ball mill at 400 rpm for 2h to obtain a uniform composite precursor.

[0116] Step Three: Structural Reconstruction and Catalytic Repair of Waste Graphite

[0117] The composite precursor was loaded into a threaded locking corundum crucible, heated to 800°C at 3°C·min⁻¹ under Ar protection, and held for 2 hours to complete the structural reconstruction and catalytic repair of the waste graphite.

[0118] Step Four: Separation of Regenerated Graphite and Metal Components

[0119] The treated composite precursor was put into a 5 mol / L NaOH solution at a liquid-solid ratio of 10 mL / g at 90°C and stirred for 1 hour to convert the liquefiable metal component into a soluble sodium salt complex; after filtration, the residue was washed with deionized water three times to obtain clean-surfaced regenerated graphite.

[0120] Step five: recycling of the metal component

[0121] The obtained filtrate was bubbled with CO2 to a pH of about 2 to form a hydroxide precipitate and precipitate Ga and other metal components; after the precipitate was collected and dried, the corresponding metal elements were obtained by adding Al powder for thermal reduction, and were recycled for the preparation of the next batch of catalyst.

[0122] The following provides test data corresponding to Example 1.

[0123] Figure 4 An XRD pattern of the regenerated graphite in an embodiment of the present disclosure is shown.

[0124] In Figure 1 In the example, the regenerated graphite obtained was subjected to XRD testing, and each diffraction peak in the pattern was clearly distinguishable and could be attributed to a hexagonal crystal structure of the P63 / mmc space group, and no impurity peaks were observed, indicating good crystallinity and phase purity. Among them, the (002) crystal face diffraction peak intensity was high, reflecting that the material had a high degree of graphitization.

[0125] Figure 5 An SEM image of the regenerated graphite in an embodiment of the present disclosure is shown.

[0126] In Figure 5 In the example, the morphology of the repaired regenerated graphite material was analyzed by SEM, and it was observed that the repaired regenerated graphite presented a typical layered structure, and the surface was relatively smooth and no obvious defects were observed, indicating that the material structure had been effectively repaired.

[0127] Figure 6 A first cycle charge-discharge curve of the regenerated graphite in an embodiment of the present disclosure at 0.1C is shown.

[0128] The electrochemical performance of the synthetic material was characterized using a 5-button battery. Regenerated graphite, conductive agent acetylene black and binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 8:1:1, and an appropriate amount of N-methyl pyrrolidone (NMP) was added as a solvent, and stirred to form a uniform slurry; the slurry was coated on a copper foil, dried at 80°C for 6 hours under vacuum, and then cut into a 12mm diameter disc as the negative electrode sheet. The electrolyte was 1mol / L LiPF6 dissolved in a mixture of EC: DMC: DEC (volume ratio 1:1:1), the separator was Celgard 2500 microporous polypropylene membrane, and lithium sheet was the counter electrode. After completing the battery assembly in an argon-filled glove box, the battery was tested at room temperature in the voltage range of 0-2V by constant current charge and discharge test (1C=372mAh・g⁻¹).

[0129] As shown in Figure 6 , the first discharge specific capacity of the battery made of regenerated graphite at 0.1C was 391mAh・g⁻¹, and the first cycle coulombic efficiency was 84.6%.

[0130] Figure 7 The long cycle curve of regenerated graphite at 1.0C in an embodiment of the present disclosure is shown.

[0131] As shown in Figure 7 , at a rate of 1C, the maximum reversible capacity of the material reached 342mAh・g⁻¹, and after 300 cycles it remained at 330mAh・g⁻¹, with a capacity retention rate of up to 96.5%, indicating that it has good electrochemical performance and meets the relevant industry standards.

[0132] Example Two

[0133] Step One: Preparation of Ni-EGaIn Graphite Catalyst

[0134] 0.20g of nickel powder and 1.00g of liquefiable metal EGaIn (mass ratio 1:5) were weighed and added to 20mL of isopropyl alcohol to form an initial dispersion system; 0.5mL of 0.1mol・L⁻¹ hydrochloric acid solution was added dropwise to the system to remove the surface oxide film of EGaIn; then under a nitrogen protective atmosphere, a probe-type ultrasonic instrument with a power of 400W was used for pulse ultrasonic treatment (10s on-1s off, for 30 minutes) to obtain a uniform and stable Ni-EGaIn semi-liquefiable metal catalyst dispersion system.

[0135] Step Two: Preparation of Graphite Catalyst Coated Graphite Composite Precursor

[0136] 2.00 g of waste graphite powder was added to the graphite catalyst, and 30 mM ethyl-3-mercaptopropionate was added as a surface activator. The mixture was then ultrasonically dispersed for 30 minutes. After filtration, the mixture was vacuum dried at 100°C for 2 hours to obtain a composite precursor of graphite coated with the catalyst.

[0137] Step 3: Structural Repair and Conductive Network Reconstruction of Waste Graphite

[0138] The composite precursor was transferred into a quartz threaded crucible and heated to 700°C at 3°C·min⁻¹ under an Ar atmosphere, and held for 2 hours to complete the graphite structure repair and conductive network reconstruction.

[0139] Step 4: Leaching and Separation of Metal Components

[0140] After repair, the material was transferred to a flask, and 100 mL of 2 mol·L⁻¹ hydrochloric acid was added. The mixture was stirred and leached at 60°C for 1 hour. At the same time, 1 mL of 30 wt% hydrogen peroxide (H₂O₂) and 0.02 mol·L⁻¹ sodium sulfite (Na₂SO₃) were added to synergistically regulate the redox environment. After leaching, the pH was adjusted to 1.5.

[0141] Step 5: Solvent extraction and recycling of metal components

[0142] Add a 1:1 volume ratio of D2EHPA-kerosene system to the obtained filtrate, and extract by shaking at room temperature for 5 minutes to allow Ga³⁺ and In³⁺ to enter the organic phase, while Ni²⁺ remains in the aqueous phase; recover the metals by back-extraction and electroreduction to complete the catalyst ring closure.

[0143] The test results are basically similar to those in Example 1. Please refer to the experimental results in Example 1. We will not go into further detail here.

[0144] Example 3

[0145] Step 1: Preparation of cobalt-EGaIn graphite catalyst

[0146] Take 0.20 g of Co(NO3)2·6H2O and dissolve it in 20 mL of ethylene glycol, add 0.80 g of liquefiable metal EGaIn; add 0.5 mL of 0.1 mol·L⁻¹ hydrochloric acid to break the surface oxide film of EGaIn, and add 30 mM ethyl-3-mercaptopropionate (EtMP) to improve interfacial wettability and droplet dispersion stability; under nitrogen protection, use a probe-type ultrasonic instrument with a power of 400 W to process for 30 minutes in pulse mode (10 seconds working, 1 second intermittent); place the obtained transition metal and liquefiable metal mixture in an inert atmosphere (Ar or N2) and heat at 100°C to remove the solvent; heat the dried solid sample in an atmosphere containing 5 vol% H2 to 500°C at a rate of 3°C·min⁻¹ and keep it for 1 hour to reduce Co(NO3)2 in situ to metallic Co, forming a graphite catalyst with cobalt particles stably dispersed in the EGaIn liquefiable metal.

[0147] Step three: preparation of composite precursor

[0148] Take 2.00 g of pretreated waste graphite powder and mix it with the above cobalt-EGaIn catalyst according to a mass ratio of 7:3, and grind it in a ball mill at a speed of 300 rpm for 2 hours to obtain a uniformly coated composite precursor; dry the obtained sample in a vacuum at 80°C for 2 hours and reserve it for use.

[0149] Step four: structure repair and conductive network reconstruction of waste graphite

[0150] Put the dried sample into a quartz threaded crucible, heat it to 750°C at a rate of 3°C·min⁻¹ in a 5 vol% H2 / Ar mixed atmosphere and keep it for 2 hours to repair the graphite structure and reconstruct the conductive network.

[0151] Step five: hot-state centrifugal separation and graphite purification

[0152] After the heat treatment is completed, cool the reaction system to 65°C and keep it for 10 minutes to allow the EGaIn to re-melt; then immediately perform a hot-state centrifugal separation operation (3500 rpm, 5 min) to separate the metal components from the graphite using the density difference; soak the upper layer graphite product in a 0.1 mol·L⁻¹ dilute sulfuric acid solution for 30 min, and after sufficient water washing and drying, obtain the regenerated graphite material.

[0153] The test results are basically similar to those of Example One, and the experimental results in Example One can be referred to, and will not be described in detail here.

[0154] In summary, the control variables in the above embodiments mainly include the types and mass ratios (1:10, 1:5, etc.) of transition metals (Ni, Co) and liquefiable metals (Ga, EGaIn), the preparation methods of catalysts (powder grinding solid-phase method, salt-ultrasonic liquid-phase method), the dispersion medium (ethanol, isopropyl alcohol, ethylene glycol), the addition of surface active agent (ethyl-3-mercapto propionate) and acid solution (hydrochloric acid), the ultrasonic parameters (power 400W, 10s on-1s off pulse mode), the ball milling parameters (rotation speed 300-400rpm, time 2h), the heat treatment atmosphere (Ar, 5vol%H2 / Ar), the heating rate (3℃・min⁻¹), the holding temperature (700-800℃) and time (2h), the separation process (alkali dissolution and acid precipitation, acid leaching and solvent extraction, hot centrifugation) and the supporting parameters (alkali / acid concentration, treatment temperature, liquid-solid ratio, centrifugal speed 3500rpm).

[0155] The purposes of setting these variables are as follows: Ni and Co are selected because of their strong catalytic activity, and Ga, EGaIn and other liquefiable metals are used because of their low melting point, high flowability and wettability, and the mass ratio is adjusted to optimize the solid-liquid contact efficiency; the powder grinding method is suitable for elemental metal powder, and the salt-ultrasonic liquid-phase method is beneficial to the uniform dispersion and in-situ reduction of soluble metal salt, thereby improving the catalytic uniformity; the dispersion medium and surface active agent enhance the wetting and adsorption of the catalyst on the surface of the waste graphite, and the acid solution can remove the oxide film on the surface of the liquefiable metal; the ultrasonic and ball milling parameters ensure that the catalyst and waste graphite are fully compounded; the heat treatment atmosphere can prevent graphite oxidation and promote catalytic reduction, the heating rate and temperature and humidity parameters balance the structure repair effect and energy consumption, and the graphite layer structure is prevented from being embrittled; different separation processes are suitable for different catalyst systems, and the supporting parameters ensure efficient recovery of metal components and high purity of regenerated graphite, thereby ultimately achieving the purposes of structure repair, improvement of electrochemical performance and closed-loop recovery of the catalyst of waste graphite.

[0156] In yet another embodiment of the present disclosure, a regenerated graphite material is provided, which is prepared by any one of the above preparation methods.

[0157] Figure 4 In the regenerated graphite, the diffraction peaks all belong to the hexagonal structure of the P63 / mmc space group, there is no impurity peak, the crystallinity and phase purity are high, the (002) crystal face diffraction peak intensity is significantly high, the graphitization degree is excellent, and the defects such as interlayer peeling and structural disorder of the waste graphite are effectively restored.

[0158] Figure 5 In the regenerated graphite, the regenerated graphite presents a typical close layer structure, the surface is smooth and flat, and there is no obvious crack and damage, and the structural integrity is comparable to that of fresh graphite.

[0159] Figure 6In the application, the first discharge specific capacity of the battery negative electrode material can reach 391 mAh g-1 at 0.1C rate, and the first cycle coulombic efficiency is not less than 84.6%.

[0160] Figure 7 In the application, the maximum reversible capacity can reach 342 mAh g-1 at 1.0C high rate, and the capacity retention rate is still as high as 96.5% after 300 cycles, and the charge-discharge reversibility, cycle stability and rate performance are excellent.

[0161] In another embodiment of the present disclosure, the use of the regenerated graphite material is provided, wherein the regenerated graphite material is used as a negative electrode material of a battery.

[0162] The regenerated graphite material can be widely used as a battery negative electrode material in lithium ion batteries (adapted to consumer electronics, power batteries, energy storage batteries, etc.), sodium ion batteries (especially suitable for large-scale energy storage scenarios), lithium-sulfur batteries, lithium ion capacitors and other new energy storage devices. The regenerated graphite material not only can significantly reduce the raw material cost of the battery negative electrode material, but also can realize the recycling of waste graphite resources, reduce resource waste and environmental pressure, has economic value and social benefits, and has broad industrial application prospects.

[0163] The above embodiments only exemplarily illustrate the principles and effects of the present disclosure, and are not used to limit the present disclosure. Any person skilled in the art can modify or change the above embodiments without departing from the spirit and scope of the present disclosure. Therefore, all equivalent modifications or changes completed by those skilled in the art without departing from the spirit and technical thought of the present disclosure should be covered by the protection scope of the present disclosure.

Claims

1. A method for preparing recycled graphite material, characterized in that, Used in the negative electrode of batteries, including: The graphite catalyst is mixed with waste graphite to obtain a composite precursor; the graphite catalyst is composed of transition metals and liquefiable metals; the melting point of the liquefiable metals is lower than that of the waste graphite. The composite precursor is placed in a preset gas environment and heated to liquefy the graphite catalyst and allow it to flow and infiltrate into the waste graphite, thereby repairing the graphite structure of the waste graphite and obtaining recycled graphite for making the negative electrode of the battery.

2. The preparation method according to claim 1, characterized in that, The transition metal includes at least one or more of Ni, Co, and Fe; and / or, The liquefiable metal includes at least elemental Ga, In, and Sn, as well as their corresponding liquid eutectic alloys.

3. The preparation method according to claim 1, characterized in that, The preparation method of the graphite catalyst includes: In an inert atmosphere, the powder of the transition metal and the liquefiable metal are mixed at a preset mass ratio to obtain a first mixture; The first mixture is ground to coat the surface of the transition metal with the liquefiable metal to form the graphite catalyst. or, The liquefiable metal is dispersed in a polar solvent and then the oxide layer of the liquefiable metal is removed by an acidic solvent to obtain the liquefiable metal to be reacted. The transition metal is added to the liquefiable metal to be reacted in a predetermined ratio, and then subjected to ultrasonic treatment in an inert or reducing atmosphere to allow the transition metal to bind to the liquefiable metal to be reacted, thereby obtaining the graphite catalyst.

4. The preparation method according to claim 1, characterized in that, The composite precursor is obtained by mixing the graphite catalyst with waste graphite and then using dry ball milling or wet ultrasonication to impregnate the surface of the waste graphite with the graphite catalyst.

5. The preparation method according to claim 1, characterized in that, The composite precursor is heated in a closed, high-temperature reaction vessel.

6. The preparation method according to claim 1, characterized in that, The preset gas environment includes an inert gas or a reducing gas; the heating treatment includes heating to 500–900°C at a rate of 2–5°C·min⁻¹ and holding at that temperature for 1–3 hours.

7. The preparation method according to claim 1, characterized in that, The recycled graphite is used to form the negative electrode of the battery.

8. The preparation method according to claim 1, characterized in that, After repairing the graphite structure of the waste graphite, the method further includes separating the regenerated graphite and the transition metal and liquefiable metal used to prepare the graphite catalyst by at least one of the following methods: alkaline dissolution and acid precipitation, acid leaching and solvent extraction, and physical separation.

9. A recycled graphite material, characterized in that, Prepared by the preparation method described in any one of claims 1-8.

10. The use of the recycled graphite material according to claim 9, characterized in that, Used as a negative electrode material in batteries.

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