A method for preparing single-layer or few-layer graphene using expanded graphite as raw material

Abstract

This invention belongs to the field of waste lithium-ion battery recycling, and more specifically, relates to a method for preparing single-layer or few-layer graphene using expanded graphite as raw material. Expanded graphite refers to expanded graphite and / or recycled graphite separated from lithium-ion battery anode materials. The expanded graphite is oxidized through a simple oxidation step to obtain graphite oxide, which is then ultrasonically dispersed and subjected to high-temperature spray pyrolysis to directly prepare few-layer or single-layer graphene. This technology has a simple process flow, reduces the use of chemical reagents, has low cost, and offers controllable quality, making it suitable for large-scale industrial production.

Description

Technical Field

[0001] This invention belongs to the field of waste lithium-ion battery recycling, and more specifically, relates to a method for preparing single-layer or few-layer graphene using expanded graphite as raw material. Background Technology

[0002] Lithium-ion batteries have gradually gained a significant position in consumer electronics, new energy vehicles, and energy storage industries in recent years due to their advantages such as high capacity, high energy density, high operating voltage, low self-discharge, good cycle stability, low cost, and environmental friendliness. However, with the explosive growth in the use of lithium-ion batteries, a large number of waste lithium-ion batteries are bound to be generated. If these waste lithium-ion batteries are not properly disposed of, they will cause significant environmental damage. Graphite is the negative electrode active material in lithium-ion batteries, accounting for approximately 20% of the battery composition. It is estimated that by 2025, more than 52 million kg of waste lithium-ion battery negative electrode materials will need to be recycled. While negative electrode graphite itself is a recyclable resource that can generate economic value again, it contains a large amount of harmful electrolytes and heavy metals. Improper disposal will lead to resource waste and ecological damage.

[0003] Large-area graphene films can be fabricated using methods such as chemical vapor deposition (CVD) and epitaxial growth, but these methods are costly, limiting their commercial application. Currently, the more widely used chemical method involves first oxidizing graphite to graphite oxide, increasing the interlayer spacing, then exfoliating to obtain graphene oxide, and finally reducing it to obtain the high-value product, graphene. The main methods for graphite oxidation include... The methods include the Saudenmaier method (using a mixture of potassium chlorate and concentrated nitric acid as the oxidant), the Saudenmaier method (using concentrated sulfuric acid and fuming nitric acid as the oxidant), and the Hummers method (using a mixed solution of concentrated sulfuric acid, sodium nitrate, and potassium permanganate as the oxidant). The reaction conditions are harsh. The Saudenmaier method does not oxidize graphite sufficiently, while the Hummers method has become the main method for preparing graphene oxide due to its high oxidation efficiency and short time consumption. However, it still requires a large amount of reducing agent and high temperature and high pressure conditions for subsequent reduction to prepare graphene.

[0004] There are currently reports on the Hummers method for regenerating graphene from waste lithium-ion battery anode graphite. CN111384462 A discloses a method for recovering graphite from waste lithium-ion battery anode materials to prepare graphene. The method involves disassembling, separating, solvent-immersing and washing, and drying the waste lithium-ion batteries to obtain crude anode graphite. This crude graphite is then extracted and washed with a eutectic solvent, followed by magnetic stirring, ultrasonic vibration, filtration, and drying to obtain eutectic solvent-extracted graphite. Finally, graphene is prepared by the Hummers method using concentrated sulfuric acid, potassium persulfate, and phosphorus pentoxide as oxidants and hydrogen peroxide as the reducing agent. This method results in a lower residual metal content in the anode carbon material after eutectic solvent extraction and washing, and a higher graphite recovery rate. However, it requires a variety of reagents and the high cost of the eutectic solvent makes it unsuitable for large-scale production. Furthermore, phosphorus pentoxide, one of the oxidant components, is highly corrosive and toxic, posing a significant health risk.

[0005] CN107879332A discloses a method for preparing graphene by spatiotemporal synchronous ultrasonic ball milling, in which graphite and an exfoliating agent are placed together in an ultrasonic ball milling device. The exfoliating agent can be any one of N-methylpyrrolidone, isopropanol, acetone, chloroform, fluorinated aromatic hydrocarbons, octafluorotoluene, pentafluorophenylcyanide, pentafluoropyrimidine, chloroform, cyclopentanone, dimethyl sulfoxide, tetrahydrofuran, γ-butyrolactone, or 1,3-dimethyl-2-imidazolinone. Graphene is prepared by ultrasonic and ball milling exfoliation. The disadvantages of this method are high ball milling speed, high energy consumption, and high ball and graphite loss, which are not conducive to large-scale applications. CN11438996A discloses a method for preparing graphene by explosive intercalation of graphite, in which a binary graphite intercalation compound is placed in a nitromethane solvent, and sufficient carbon dioxide is injected as a protective medium. An explosive is ignited, and a large amount of gas is generated during the explosion to exfoliate the graphite. This process has poor safety, and the short duration of the explosive impact will lead to insufficient graphite exfoliation. Furthermore, CN110127679A discloses a ball milling method for preparing graphene by intercalating nitromethane with graphite, in which the binary graphite intercalation compound and dry ice are placed together in a sealed ball mill. During the ball milling process, the heat of nitromethane explosion decomposition, the gas, and the ball milling work together to exfoliate the graphite. This process combines explosion and ball milling to ensure sufficient graphite exfoliation. While the above methods are simple, they all pose a safety hazard due to the large explosive impact force, making them unsuitable for large-scale production applications.

[0006] Single-layer graphene has significant advantages over multilayer graphene in application performance, such as higher electron mobility, thermal conductivity, light transmittance, mechanical properties, thermal conductivity, and energy conversion efficiency. Graphene prepared using existing technologies is often multilayer graphene, and it is difficult to prepare single-layer or few-layer graphene with fewer than two layers.

[0007] In summary, existing technologies for preparing graphene from graphite suffer from problems such as high cost, long processing time, poor safety, low graphite exfoliation degree, and difficulty in obtaining single-layer or few-layer graphene. Therefore, developing a green process and technology for preparing single-layer or few-layer graphene with low reagent consumption, simple process, and high safety is worthy of attention and research. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing monolayer or few-layer graphene using expanded graphite as a raw material. This invention simplifies the oxidation reaction process of waste graphite, then directly disperses the oxidized graphite via ultrasonication followed by high-temperature spray pyrolysis to obtain near-monolayer graphene. The process is simple and requires no additional reducing agent. Here, the expanded graphite can be either expanded graphite or recycled graphite separated from lithium-ion battery anode materials. This technology not only alleviates the environmental pressure of waste lithium-ion batteries and conserves graphite resources, but also enables the preparation of monolayer or few-layer graphene, resulting in significant economic benefits and profound implications for the sustainable development of the lithium-ion battery industry.

[0009] To achieve the above objectives, the present invention provides a method for preparing single-layer or few-layer graphene using expanded graphite as a raw material, comprising the following steps:

[0010] (1) Expanded graphite is oxidized with an oxidizing agent to form graphite oxide; the expanded graphite is expanded graphite and / or recycled graphite separated from lithium-ion battery negative electrode materials; the oxidizing agent includes concentrated sulfuric acid and potassium permanganate; the oxidation is specifically: the expanded graphite is mixed with a mixed oxidizing agent of concentrated sulfuric acid and potassium permanganate and reacted first at an ice-water bath temperature, and then the temperature is raised to a medium temperature of less than or equal to 70°C to carry out the oxidation reaction to obtain graphite oxide;

[0011] (2) The graphene oxide is ultrasonically dispersed in a solvent and then prepared by high-temperature spray pyrolysis to obtain single-layer or few-layer graphene.

[0012] Preferably, the method for obtaining the recycled graphite includes the following steps: separating the disassembled lithium-ion battery anode material by soaking it in an organic solvent to obtain waste graphite; and removing impurities from the waste graphite by acid washing and drying to obtain the purified recycled graphite, which is the recycled graphite separated from the lithium-ion battery anode material.

[0013] Preferably, the lithium-ion battery is one or more of the following lithium-ion batteries with lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium iron phosphate as the positive electrode material.

[0014] Preferably, the organic solvent is one or more of methanol, ethanol, isopropanol, acetone, N-methylformamide, N-methylpyrrolidone, dimethylacetamide, triethyl phosphate, and ionic liquids (such as 1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-2,3-dimethylpyridinium salt, tetraethylammonium salt, trioctyltetradecylphospholipidium salt, glycineium salt, 1-butyl-3-methylamidinium salt, etc.).

[0015] Preferably, the solid-liquid ratio of the disassembled lithium-ion battery negative electrode material to the organic solvent is 1:10-1:100 g / mL, the immersion temperature is 20-60℃, and the time is 5-60 min.

[0016] Preferably, the impurity removal reagent used in the acid washing is a Lewis acid, and more preferably one or more of citric acid, formic acid, hydrochloric acid, hydrofluoric acid, and trifluoromethanesulfonic acid.

[0017] Preferably, the concentration of the Lewis acid is 0.05-3 mol / L; the solid-liquid ratio of the waste graphite to the Lewis acid is 1:10-1:200 g / mL, the reaction time is 10-90 min, and the temperature is 30-90℃.

[0018] Preferably, the oxidation in step (1) is as follows: the expanded graphite is mixed with a mixed oxidant of concentrated sulfuric acid and potassium permanganate and reacted at an ice-water bath temperature for 60-180 min, and then the temperature is raised to a medium temperature of 30-70℃ for 4-10 h.

[0019] More preferably, the oxidation in step (1) is as follows: the expanded graphite is mixed with a mixed oxidant of concentrated sulfuric acid and potassium permanganate and reacted at an ice-water bath temperature for 60-90 min, and then the temperature is raised to a medium temperature of 40-60℃ and reacted for 7-9 h.

[0020] Preferably, the ratio of expanded graphite, concentrated sulfuric acid and potassium permanganate is 1g:(15-25)mL:(1-5)g.

[0021] Preferably, the ultrasonic dispersion power is 150W-270W; the ultrasonic dispersion time is 10-60min.

[0022] Preferably, the solvent is one or more selected from water, isopropanol, N-methylformamide, methanol, and ethanol.

[0023] Preferably, the high-temperature spray pyrolysis has a pyrolysis temperature of 100-500℃, and more preferably 150-300℃.

[0024] In summary, compared with the prior art, the above-described technical solutions conceived by this invention mainly possess the following technical advantages:

[0025] (1) This invention discovers through experiments that recycled graphite separated from lithium-ion battery anode materials may have a similar structure to expanded graphite. In this invention, both are collectively referred to as expanded graphite. Expanded graphite is combined with a simplified oxidation process proposed in this invention to produce graphite oxide. After ultrasonication, high-temperature spray pyrolysis can prepare single-layer or few-layer graphene. During the chemical oxidation of expanded graphite, the interlayer spacing increases due to the introduction of oxygen-containing functional groups. Then, high-temperature spray pyrolysis technology, which combines exfoliation and reduction, is used to exfoliate and reduce the graphite oxide to graphene, thereby regenerating waste graphite into high-value-added near-single-layer graphene products. This technology has a simple process flow, reduces the use of chemical reagents, has low cost, and controllable quality, making it suitable for large-scale industrial production.

[0026] (2) This invention separates and recovers high-purity graphite from waste lithium-ion batteries using a mixed oxidant of concentrated sulfuric acid and potassium permanganate. During the oxidation process, graphite oxide with a high degree of oxidation can be prepared through low-temperature and medium-temperature reactions, and the process is short. Then, the graphite oxide is ultrasonically dispersed and then subjected to high-temperature spray pyrolysis to prepare single-layer or few-layer graphene. In the experiment, the graphite recovered from waste batteries in this invention was replaced with flake graphite or expanded graphite. When using the oxidation-ultrasonic-high-temperature spray pyrolysis process of this invention, it was found that only graphene with 34 layers could be obtained using flake graphite, but using expanded graphite, like the graphite recovered from lithium-ion batteries in this invention, could obtain near-single-layer graphene. However, if the oxidation process is replaced by a three-step oxidation process similar to the traditional Hummers oxidation method, such as a low-temperature-medium-temperature oxidation process in the embodiments of the present invention, then flake graphite, commercially available expanded graphite, and recycled graphite in the embodiments of the present invention cannot produce near-monolayer graphene. This shows that the improved oxidation process of the present invention is crucial for obtaining near-monolayer graphene from expanded graphite. Moreover, graphite materials recycled from waste lithium-ion batteries may have a structure similar to expanded graphite due to the influence of battery charging and discharging. Compared with ordinary flake graphite materials, they are more suitable for using the oxidation-ultrasonic-high-temperature spray pyrolysis process of the present invention to produce monolayer or few-layer graphene. Attached Figure Description

[0027] Figure 1 A schematic flowchart of a method for separating graphite from waste lithium-ion battery anode materials and preparing graphene provided by the present invention;

[0028] Figure 2 Thermogravimetric curve of graphite oxide prepared in Example 1 under air conditions;

[0029] Figure 3 This is a schematic diagram illustrating the changes in the microstructure of graphite during the preparation process of this invention.

[0030] Figure 4The FTIR spectrum of graphite oxide prepared in Example 1;

[0031] Figure 5 Comparison of XRD patterns of graphene prepared in Example 1 and commercial graphene;

[0032] Figure 6 SEM comparison images of graphene prepared in Example 1 and commercial graphene;

[0033] Figure 7 The Raman spectrum of the graphene prepared in Example 1;

[0034] Figure 8 The Raman spectrum of the graphene prepared in Example 3;

[0035] Figure 9 The Raman spectrum of the graphene prepared in Example 4;

[0036] Figure 10 The XRD pattern of graphite oxide prepared in Comparative Example 1;

[0037] Figure 11 SEM images of the graphene prepared in comparative examples 1 to 3;

[0038] Figure 12 The Raman spectrum of the graphene prepared in Comparative Example 4 is shown.

[0039] Figure 13 Raman spectra of graphene prepared in Comparative Example 5;

[0040] Figure 14 The FTIR spectrum of graphene oxide prepared in Comparative Example 6;

[0041] Figure 15 The Raman spectrum of the graphene prepared in Comparative Example 6;

[0042] Figure 16 The FTIR spectrum of graphite oxide prepared in Comparative Example 7;

[0043] Figure 17 The Raman spectrum of the graphene prepared in Comparative Example 7;

[0044] Figure 18 The Raman spectrum of the graphene prepared in Comparative Example 8;

[0045] Figure 19 Raman spectra of graphene prepared in Comparative Example 9;

[0046] Figure 20 The Raman spectrum of the graphene prepared in Comparative Example 10 is shown. Detailed Implementation

[0047] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Those skilled in the art should understand that the embodiments described are merely illustrative of the invention and should not be considered as specific limitations thereof. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0048] The embodiments of the present invention are implemented under the premise of the technical solution of the present invention, and detailed implementation methods and processes are given. However, the protection scope of the present invention is not limited to the following embodiments. The process parameters in the following embodiments that do not specify specific conditions are generally in accordance with conventional conditions.

[0049] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0050] This invention provides a method for preparing single-layer or few-layer graphene using expanded graphite as a raw material, comprising the following steps:

[0051] (1) Expanded graphite is oxidized with an oxidizing agent to form graphite oxide; In this invention, expanded graphite and recycled graphite separated from lithium-ion battery negative electrode materials are collectively referred to as expanded graphite; The oxidizing agent used in the oxidation process of this invention includes concentrated sulfuric acid and potassium permanganate; The oxidation process is specifically as follows: the expanded graphite is mixed with the mixed oxidizing agent of concentrated sulfuric acid and potassium permanganate and reacted first at an ice-water bath temperature, and then the temperature is raised to a medium temperature of less than or equal to 70°C to carry out the oxidation reaction to obtain graphite oxide;

[0052] (2) The graphene oxide is ultrasonically dispersed in a solvent and then prepared by high-temperature spray pyrolysis to obtain single-layer or few-layer graphene.

[0053] The single-layer or few-layer graphene described in this invention refers to graphene with fewer than or equal to 2 layers, preferably fewer than 1.5 layers.

[0054] In a preferred embodiment, the oxidation is performed by mixing expanded graphite (i.e., expanded graphite and / or recycled graphite) with a mixed oxidant of concentrated sulfuric acid and potassium permanganate, reacting it first at an ice-water bath temperature for 60-180 min, and then raising the temperature to a medium temperature of 30-70°C for 4-10 h.

[0055] In the traditional Hummers process for preparing graphite oxide, high-purity graphite is typically first mixed with concentrated sulfuric acid and subjected to a low-temperature (ice-water bath) – medium-temperature (32℃-38℃) – high-temperature (>90℃) oxidation reaction. Potassium permanganate is added during the ice-water bath reaction to prepare graphite oxide. Concentrated sulfuric acid acts as an intercalating agent, first penetrating the edges of graphite sheets or defects in the graphite structure and oxidizing them. Polar sulfuric acid molecules and hydrogen sulfate ions gradually insert into the graphite structural domains through intermolecular forces and electrostatic attraction, forming sulfuric acid-graphite intercalation compounds. The reaction equation is: H₂SO₄ + G + [O] → H₂O + [G + HSO₄] - [·H2SO4]. Then potassium permanganate is added; its ionized oxygen-containing functional groups enter the interlayer along the edges of the graphite sheets, further oxidizing the sulfuric acid-graphite interlayer compound. The reaction equation is: MnO4 - +[G+HSO4 - [·H₂SO₄]→MnO₂+SO x +H₂O+GO, the oxidation process increases the interlayer spacing of graphite, and its hybridization mode changes from sp 2 Transform into sp 3 .

[0056] This invention relates to the oxidation of expanded graphite (i.e., expanded graphite or high-purity graphite recovered from waste lithium-ion battery anode materials) to prepare graphite oxide. The oxidants, concentrated sulfuric acid and potassium permanganate, are simultaneously mixed with the expanded graphite, and oxidation reactions are carried out at low temperature (ice-water bath) and medium temperature (30-70℃), respectively, to prepare high-purity graphite. Unlike the traditional Hummers process, this invention involves the simultaneous reaction of concentrated sulfuric acid and potassium permanganate with high-purity graphite. Under the action of the mixed oxidant system, significant intercalation and oxidation reactions occur, releasing heat. The graphite lattice expands and / or exfoliates, and a large number of oxygen-containing functional groups are gradually incorporated. As the reaction time in the medium-temperature stage increases, the graphite phase gradually decreases until it disappears. The reaction equation is: H₂SO₄ + MnO₄ - +G→GO+MnSO4+O2+H2O. The graphite oxidation mechanism of this invention differs significantly from the Hummers method, allowing for the direct one-step synthesis of graphite oxide (GO), thus improving reaction efficiency.

[0057] Existing Hummers processes or modified Hummers processes use concentrated sulfuric acid and potassium permanganate as oxidants to prepare graphite oxide. The concentrated sulfuric acid and potassium permanganate are introduced sequentially, not simultaneously. First, concentrated sulfuric acid and graphite are mixed and oxidized at a low temperature. During the low-temperature reaction, potassium permanganate is added to continue the oxidation reaction. Then, the temperature is raised to a medium temperature, and finally to a high temperature to continue the oxidation reaction, thus obtaining graphite oxide. However, this invention directly mixes both oxidants simultaneously with recycled battery graphite or expanded graphite, omitting the high-temperature reaction step. Experiments have shown that the graphite oxide prepared in this way can be used to prepare few-layer or single-layer graphene using the ultrasonic-high-temperature spray pyrolysis method of this invention. Furthermore, it avoids the technical defect of manganese dioxide impurities in the graphene product caused by the stepwise introduction of concentrated sulfuric acid and potassium permanganate. The possible reason is that the present invention uses recycled graphite obtained by separating and removing impurities from lithium-ion battery anode materials as raw material to prepare graphene through oxidation, exfoliation and reduction. It is speculated that the charging and discharging process of lithium-ion batteries has a certain amplification effect on the interlayer spacing of graphite, which makes it easier to prepare single-layer or few-layer graphene according to the method of the present invention, similar to expanded graphite, compared with ordinary graphite such as flake graphite.

[0058] In step (1), since a large amount of heat will be released during the oxidation reaction, the oxidation reaction is first carried out in an ice-water bath (0°C) for 60-180 min and then at a medium temperature of 30-70°C for 4-10 h. The preferred reaction time in the ice-water bath is 60-90 min, the medium temperature is 40-60°C, and the medium temperature reaction time is 7-9 h.

[0059] In some embodiments, the ratio of expanded graphite, concentrated sulfuric acid and potassium permanganate is 1g:(15-25)mL:(1-5)g, and more preferably 1g:(20-25)mL:(2-4)g.

[0060] In some embodiments, hydrogen peroxide is added to the system after the oxidation reaction is complete to consume any unreacted potassium permanganate during the reaction.

[0061] In some embodiments, the power of ultrasonic dispersion in step (2) is 150W-270W, preferably 200W-250W; the ultrasonic dispersion time is 10-60min, preferably 20-40min. This invention ultrasonically disperses graphite oxide in a solvent, facilitating subsequent spray pyrolysis. During the ultrasonic dispersion process, graphite oxide undergoes exfoliation to form graphene oxide.

[0062] In some embodiments, the solvent in step (2) is one or more of water, isopropanol, N-methylformamide, methanol, and ethanol.

[0063] In some embodiments, the high-temperature spray pyrolysis in step (2) has a pyrolysis temperature of 100-500℃, preferably 150-300℃.

[0064] In some embodiments, a general-purpose spray pyrolysis dryer or similar equipment is used. The spray pyrolysis temperature is expressed as the inlet temperature, which is set to 100-500°C, preferably 150-300°C. The liquid spraying rate is 10-200 ml / h, preferably 30-100 ml / h, and more preferably 40-60 ml / h. The liquid spraying rate determines the residence time of the material in the spray pyrolysis equipment.

[0065] In some embodiments, obtaining the recycled graphite includes the following steps: separating the disassembled lithium-ion battery negative electrode material by soaking it in an organic solvent to obtain waste graphite; and removing impurities from the waste graphite by acid washing and drying to obtain purified recycled graphite.

[0066] Figure 1 This is a schematic flowchart illustrating a method for separating graphite from waste lithium-ion battery anode materials and preparing single-layer or few-layer graphene, provided in some embodiments of the present invention. Specifically, the method includes the following steps:

[0067] (1) After the waste lithium-ion battery negative electrode material is shredded, it is mixed with an organic solvent at a certain solid-liquid ratio. The mixture is stirred magnetically and filtered after a certain reaction time to obtain waste graphite.

[0068] (2) The waste graphite obtained in step (1) is vacuum dried and then acid leaching is used to remove the metal impurities in it to obtain the cleaned recycled graphite.

[0069] (3) Prepare graphite oxide by chemical oxidation of the recycled graphite described in step (2) using an oxidant;

[0070] (4) The graphene oxide described in step (3) is ultrasonically dispersed into the liquid phase and then prepared by high-temperature spray pyrolysis. Here, ultrasonication and high-temperature spray pyrolysis are used in combination. First, the graphene oxide is uniformly dispersed into the liquid solvent by ultrasonication. Under this action, the graphene oxide is peeled off into graphene oxide. Then, it is introduced into a high-temperature spray pyrolysis device. Due to the acceleration effect, the organic solvent containing graphene oxide is atomized into tiny droplets. Then, the droplets are carried in the gas flow and sent into the heating reaction furnace. Under the action of high temperature, the organic solvent in the droplets evaporates, and the oxygen-containing functional groups in the graphene oxide undergo a deoxygenation reaction and are reduced to graphene.

[0071] This invention provides a method for separating graphite from waste lithium-ion battery anode materials and preparing graphene. The method first separates waste graphite from copper foil current collectors by soaking in an organic solvent, then purifies it with Lewis acid, followed by oxidation to graphite oxide. After ultrasonic dispersion, the graphite is exfoliated and reduced to few-layer or single-layer graphene through high-temperature spray pyrolysis. This method is simple, has a short preparation cycle, low reagent consumption, low cost, and can be mass-produced, solving the problems of high cost, high energy consumption, poor safety, and low graphite exfoliation degree in existing graphene preparation methods. Furthermore, the regenerated graphene can be applied in energy storage, catalysis, and environmental pollution control, effectively improving the recycling value of graphite.

[0072] The waste lithium-ion batteries mentioned in this invention can be various lithium-ion batteries containing graphite negative electrodes, including but not limited to one or more of lithium-ion batteries with positive electrode materials such as lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium iron phosphate.

[0073] The binders used in lithium-ion battery anodes mainly include polyvinylidene fluoride, sodium carboxymethyl cellulose, polyacrylic acid, styrene-butadiene rubber, polyvinyl alcohol, and polyimide. When soaked in organic solvents, solvent molecules penetrate between the polymer chains, weakening the van der Waals forces and hydrogen bonds between polymer molecules. When the interaction force between the solvent molecules and the polymer chains is greater than the interaction force between polymer molecules, the polymer chains are broken apart, thereby dissolving the binder and efficiently separating waste graphite. Step (1) can be based on the binder material used in the negative electrode of the waste lithium-ion battery and select various organic solvents that can separate waste graphite, including but not limited to one or more polar solvents such as methanol, ethanol, isopropanol, acetone, N-methylformamide, N-methylpyrrolidone, dimethylacetamide, dimethylformamide, triethyl phosphate, ionic liquids (such as 1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-2,3-dimethylpyridinium salt, tetraethylammonium salt, trioctyltetradecylphospholipidium salt, glycinium salt, 1-butyl-3-methylamidinium salt, etc.).

[0074] In some embodiments, the solid-liquid ratio of the disassembled lithium-ion battery negative electrode material to the organic solvent is 1:10-1:100 g / mL, the immersion temperature is 20-60℃, and the time is 5-60 min. More preferably, the solid-liquid ratio is 1:10-1:50 g / L, the immersion temperature is 30-60℃, and the time is 20-40 min.

[0075] In some embodiments, the impurity removal reagent used in step (2) is a Lewis acid, including but not limited to one or more of citric acid, formic acid, hydrochloric acid, hydrofluoric acid, and trifluoromethanesulfonic acid; the concentration of the Lewis acid is 0.05-3 mol / L, the solid-liquid ratio of the waste graphite to the Lewis acid is 1:10-1:200 g / mL, the reaction time is 10-90 min, and the temperature is 30-90℃. More preferably, the concentration of the Lewis acid is 0.05-2 mol / L, the solid-liquid ratio is 1:10-1:50 g / mL, the reaction time is 20-40 min, and the temperature is 50-90℃.

[0076] In some embodiments, the recovery rate of waste graphite in step (1) reaches more than 95%, and the metal removal rate of waste graphite in the acid washing process in step (2) reaches more than 99%.

[0077] This invention uses expanded graphite or recycled graphite from the negative electrodes of spent lithium-ion batteries as raw materials. A simplified oxidation process is used to produce graphite oxide, which is then combined with ultrasonic dispersion and high-temperature spray pyrolysis to prepare single-layer or few-layer graphene. This invention employs chemical oxidation combined with high-temperature spray pyrolysis to prepare graphene, which is simpler than the Hummers process and does not require additional reducing agents. This technology not only alleviates the environmental pressure of spent lithium-ion batteries and conserves graphite resources, but also has significant economic benefits and profound implications for the sustainable development of the lithium-ion battery industry.

[0078] The flake graphite and expanded graphite used in the embodiments of this invention are all commercially available products.

[0079] The following are specific examples:

[0080] Example 1

[0081] This embodiment provides a method for separating graphite from the negative electrode sheet of a spent lithium-ion battery and preparing graphene through high-temperature spray pyrolysis. Figure 1 This embodiment provides a flowchart of a method for separating graphite from the negative electrode sheet of a spent lithium-ion battery and preparing graphene using high-temperature spray pyrolysis, including the following steps:

[0082] (1) Cut the negative electrode sheet of the waste lithium-ion battery to 1cm. 2 The graphite was mixed with triethyl phosphate at a solid-liquid ratio of 1:10 g / mL. The mixture was magnetically stirred and reacted at 20°C for 60 min. After filtration, the recovered graphite was obtained with a recovery rate of 95%.

[0083] (2) The recovered graphite obtained in step (1) was vacuum dried and then leached with citric acid to remove the metal impurities, resulting in high-purity graphite. The citric acid concentration was 0.05 mol / L, the solid-liquid ratio was 1:50 g / mL, the temperature was 90℃, and the leaching time was 40 min. The removal rate of metal impurities reached 99.5%.

[0084] (3) The high-purity graphite described in step (2) is chemically oxidized using an oxidizing agent composed of concentrated sulfuric acid and potassium permanganate to form graphite oxide. The ratio of high-purity graphite:concentrated sulfuric acid:potassium permanganate is 1g:25mL:3g. The oxidation reaction is first carried out in an ice-water bath for 90 minutes, and then at a medium temperature of 50℃ for 8 hours to obtain a golden-yellow graphite oxide solution. Hydrogen peroxide is then added until no more bubbles are generated. After standing, the solution is filtered, and finally washed with deionized water until neutral. After drying, a well-oxidized graphite oxide powder is obtained. Its FTIR infrared spectrum is as follows: Figure 4 As shown, this indicates that oxygen-containing groups such as carbonyl, hydroxyl, and epoxy groups exist in graphite oxide.

[0085] (4) After ultrasonically dispersing the graphite oxide obtained in step (3) with water at 240W for 30 min, graphene was prepared by high-temperature spray pyrolysis at 300℃. The inlet temperature was 300℃, the liquid spraying rate was 50 ml / h, and the atmosphere was air. Its XRD pattern, SEM image, and Raman spectrum are as follows: Figure 5 , Figure 6 and Figure 7 As shown in the figure, the XRD pattern reveals that its diffraction peak appears at a 2θ angle of 26°, corresponding to the (0 0 2) crystal plane of graphene. This is due to the sp... 2 This is due to the hexagonal lattice structure of hybrid carbon atoms. SEM images show that graphene has a smooth surface, exhibits a light color, and has obvious wrinkles. Raman spectroscopy results show that the G peak appears at 1585.5 cm⁻¹. -1 At that point, by formula W G =1581.6+11 / (1+n) 1.6 The calculated number of graphene layers, n, is 1.45, indicating that the method produces a relatively small number of graphene layers.

[0086] Figure 2 The thermogravimetric curve of graphite oxide prepared in Example 1 under air conditions shows that the weight loss of the sample can be divided into four stages. In the first stage (28-125℃), due to the loss of adsorbed water, the weight loss is 15.22%. In the second stage (125-189℃), due to the loss of water molecules directly bonded to oxygen-containing functional groups by hydrogen bonds, the weight loss rate is 10.71%. In the third stage (189-387℃), due to the thermal decomposition of oxygen-containing functional groups in the sample, the weight loss rate is 16.84%. Finally, an exothermic peak appears at 515.2℃, which is due to the combustion of the carbon skeleton. Therefore, the temperature for high-temperature spray pyrolysis reduction of graphite oxide should be below 500℃.

[0087] Figure 3 This is a schematic diagram of the microstructure changes of graphite during the preparation process of an embodiment of the present invention. First, the collected lithium-ion battery negative electrode graphite is separated and purified to obtain recycled graphite, which is also called expanded graphite. After chemical oxidation, graphite oxide is obtained, and then graphene is prepared by ultrasonic dispersion and high-temperature spray pyrolysis.

[0088] Figure 4 The FTIR spectrum of graphite oxide prepared in Example 1 shows that the graphite oxide prepared under these conditions contains oxygen-containing groups such as carbonyl, carboxyl, and epoxy groups, indicating that its oxidation degree is high.

[0089] Figure 5 The XRD pattern of graphene prepared in Example 1 shows that its diffraction peak appears at a 2θ angle of 26°, corresponding to the (0 0 2) crystal plane of graphene. This is due to the sp 2 This is caused by the hexagonal lattice structure of hybrid carbon atoms.

[0090] Figure 6 The SEM images of the graphene prepared in Example 1 and commercial graphene show that the prepared graphene has a smoother surface, a lighter color and more obvious wrinkles compared to commercial graphene, indicating that it has fewer layers.

[0091] Figure 7 The image shows the Raman spectrum of the graphene prepared in Example 1; it can be seen that the G peak appears at 1585.5 cm⁻¹. -1 At this point, peak D appears at 1347.1 cm. -1 At this location, the number of layers was calculated to be 1.45. The conductivity of the graphene prepared in this example was measured to be 1.66 × 10⁻⁶. 7 S / m.

[0092] Example 2

[0093] (1) Cut the negative electrode sheet of the waste lithium-ion battery to 1cm. 2 The graphite was mixed with N-methylpyrrolidone at a solid-liquid ratio of 1:100 g / mL. The mixture was magnetically stirred and reacted at 60°C for 20 min. After filtration, the graphite was recovered with a recovery rate of 99%.

[0094] (2) The recovered graphite obtained in step (1) was vacuum dried and then leached with hydrochloric acid to remove metal impurities, resulting in high-purity graphite. The hydrochloric acid concentration was 2 mol / L, the solid-liquid ratio was 1:10 g / mL, the temperature was 50℃, and the leaching time was 20 min. The removal rate of metal impurities reached 99.6%.

[0095] (3) The high-purity graphite described in step (2) is chemically oxidized using an oxidant composed of concentrated sulfuric acid and potassium permanganate to form graphite oxide. The ratio of waste graphite: concentrated sulfuric acid: potassium permanganate is 1g: 15mL: 4g. The oxidation reaction is first carried out in an ice-water bath for 90 minutes, and then at a medium temperature of 60℃ for 8 hours to obtain a golden-yellow graphite oxide solution. Hydrogen peroxide is then added until no more bubbles are generated. After standing, the solution is filtered, and finally washed with deionized water until neutral, and then dried to obtain graphite oxide.

[0096] (4) The graphene oxide obtained in step (3) was ultrasonically dispersed in ethanol at 240W for 60 min and then pyrolyzed at 250℃ to prepare graphene. The air inlet temperature was set to 250℃, the liquid spraying rate was 50ml / h, and the atmosphere was air.

[0097] Example 3

[0098] (1) Cut the negative electrode sheet of the waste lithium-ion battery to 1cm. 2 The graphite was mixed with N-methylformamide at a solid-liquid ratio of 1:50 g / mL. The mixture was stirred magnetically and reacted at 40°C for 40 min. The mixture was then filtered to obtain recycled graphite with a recovery rate of 99%.

[0099] (2) The recovered graphite obtained in step (1) was vacuum dried and then leached with sulfuric acid to remove the metal impurities, resulting in high-purity graphite. The sulfuric acid concentration was 0.5 mol / L, the solid-liquid ratio was 1:100 g / mL, the temperature was 70℃, and the leaching time was 30 min. The removal rate of metal impurities reached 99.5%.

[0100] (3) The high-purity graphite described in step (2) is chemically oxidized using an oxidant composed of concentrated sulfuric acid and potassium permanganate to form graphite oxide. The ratio of waste graphite: concentrated sulfuric acid: potassium permanganate is 1g: 20mL: 2g. The oxidation reaction is first carried out in an ice-water bath for 90 minutes, and then at a medium temperature of 50℃ for 10 hours to obtain a golden-yellow graphite oxide solution. Then, hydrogen peroxide is added until no more bubbles are generated. After standing, the solution is filtered, and finally washed with deionized water until neutral, and then dried to obtain graphite oxide.

[0101] (4) The graphene oxide prepared in step (3) was ultrasonically dispersed with N-methylformamide at 240W for 30 min and then subjected to high-temperature spray pyrolysis at 200℃ to prepare graphene oxide. The air inlet temperature was set to 200℃, the liquid spray rate was 50 ml / h, and the atmosphere was air. The Raman spectrum of the graphene prepared in Example 3 is shown below. Figure 8 As shown, the G peak appears at 1585.4 cm⁻¹. -1 At this point, peak D appears at 1353.8 cm. -1 The calculated number of floors is 1.49.

[0102] Example 4

[0103] (1) Expanded graphite was chemically oxidized using an oxidizing agent composed of concentrated sulfuric acid and potassium permanganate to form graphite oxide, with the ratio of expanded graphite:concentrated sulfuric acid:potassium permanganate = 1g:25mL:3g. The oxidation reaction was first carried out in an ice-water bath for 90 minutes, and then at a medium temperature of 50℃ for 8 hours to obtain a golden yellow graphite oxide solution. Hydrogen peroxide was then added until no more bubbles were generated. After standing, the solution was filtered, and finally washed with deionized water until neutral. The solution was then dried to obtain graphite oxide powder with a good degree of oxidation.

[0104] (2) The graphene oxide obtained in step (1) was ultrasonically dispersed in water at 240W for 30 minutes, and then prepared by high-temperature spray pyrolysis at 300℃. The inlet temperature was 300℃, the liquid spraying rate was 50ml / h, and the atmosphere was air. Raman test results. Figure 9 The result shows that peak G appears at 1585.8 cm. -1 At that point, by formula W G =1581.6+11 / (1+n) 1.6 The calculated number of graphene layers, n, is 1.35, indicating that the method produces a relatively small number of graphene layers.

[0105] Comparative Example 1

[0106] (1) Cut the negative electrode sheet of the waste lithium-ion battery to 1cm. 2 The graphite was mixed with triethyl phosphate at a solid-liquid ratio of 1:10 g / mL. The mixture was magnetically stirred and reacted at 20°C for 60 min. After filtration, the recovered graphite was obtained with a recovery rate of 95%.

[0107] (2) The recovered graphite obtained in step (1) was vacuum dried and then leached with citric acid to remove the metal impurities, resulting in high-purity graphite. The citric acid concentration was 0.05 mol / L, the solid-liquid ratio was 1:50 g / mL, the temperature was 90℃, and the leaching time was 40 min. The removal rate of metal impurities reached 99.5%.

[0108] (3) The high-purity graphite described in step (2) was chemically oxidized using an oxidant composed of concentrated sulfuric acid and potassium permanganate to form graphite oxide. The ratio of high-purity graphite: concentrated sulfuric acid: potassium permanganate was 1g:10mL:0.5g. The oxidation reaction was first carried out in an ice-water bath for 90 minutes, and then at a medium temperature of 50℃ for 8 hours to obtain a graphite oxide solution. Hydrogen peroxide was then added until no more bubbles were generated. After standing, the solution was filtered, and finally washed with deionized water until neutral. The solution was then dried to obtain graphite oxide powder. Due to insufficient dosage of the oxidant concentrated sulfuric acid and potassium permanganate, the degree of oxidation was low. It still retained the characteristic peaks of graphite, but no characteristic peaks of graphite oxide were observed. Its XRD pattern is as follows. Figure 10 As shown.

[0109] (4) After ultrasonically dispersing the graphite oxide obtained in step (3) with water at 240W for 30 minutes, graphene was prepared by high-temperature spray pyrolysis at 300℃. The air inlet temperature was set to 300℃, the liquid spraying rate was 50ml / h, and the atmosphere was air. The SEM image of the product prepared by this process is shown below. Figure 11 As shown in Comparative Example 1, it still retains the layered structure of graphite.

[0110] Comparative Example 2

[0111] (1) Cut the negative electrode sheet of the waste lithium-ion battery to 1cm. 2 The graphite was mixed with N-methylpyrrolidone at a solid-liquid ratio of 1:100 g / mL. The mixture was magnetically stirred and reacted at 60°C for 20 min. After filtration, the graphite was recovered with a recovery rate of 99%.

[0112] (2) The recovered graphite obtained in step (1) was vacuum dried and then leached with hydrochloric acid to remove metal impurities, resulting in high-purity graphite. The hydrochloric acid concentration was 2 mol / L, the solid-liquid ratio was 1:10 g / mL, the temperature was 50℃, and the leaching time was 20 min. The removal rate of metal impurities reached 99.6%.

[0113] (3) The high-purity graphite described in step (2) is chemically oxidized using an oxidant composed of concentrated sulfuric acid and potassium permanganate to form graphite oxide. The ratio of waste graphite: concentrated sulfuric acid: potassium permanganate is 1g: 15mL: 4g. The oxidation reaction is first carried out in an ice-water bath for 90 minutes, and then at a medium temperature of 60℃ for 8 hours to obtain a graphite oxide solution. Hydrogen peroxide is then added until no more bubbles are generated. After standing, the solution is filtered, and finally washed with deionized water until neutral. The solution is then dried to obtain graphite oxide.

[0114] (4) Graphene was prepared by ultrasonically dispersing the graphene oxide obtained in step (3) with ethanol at 240W for 60 min and then drying. The SEM image of the product obtained by ultrasonication without reduction is shown below. Figure 11 As shown in Comparative Example 2, it still maintains a layered structure.

[0115] Comparing the graphene prepared in Comparative Example 2 with that prepared in Example 1, it was found that ultrasonication alone, without spray pyrolysis, failed to achieve complete exfoliation and reduction of graphene oxide.

[0116] Comparative Example 3

[0117] (1) Cut the negative electrode sheet of the waste lithium-ion battery to 1cm. 2 The graphite was mixed with N-methylpyrrolidone at a solid-liquid ratio of 1:100 g / mL. The mixture was magnetically stirred and reacted at 60°C for 20 min. After filtration, the graphite was recovered with a recovery rate of 99%.

[0118] (2) The recovered graphite obtained in step (1) was vacuum dried and then leached with hydrochloric acid to remove metal impurities, resulting in high-purity graphite. The hydrochloric acid concentration was 2 mol / L, the solid-liquid ratio was 1:10 g / mL, the temperature was 50℃, and the leaching time was 20 min. The removal rate of metal impurities reached 99.6%.

[0119] (3) The high-purity graphite described in step (2) is chemically oxidized using an oxidant composed of concentrated sulfuric acid and potassium permanganate to form graphite oxide. The ratio of waste graphite: concentrated sulfuric acid: potassium permanganate is 1g: 15mL: 4g. The oxidation reaction is first carried out in an ice-water bath for 90 minutes, and then at a medium temperature of 60℃ for 8 hours to obtain a graphite oxide solution. Hydrogen peroxide is then added until no more bubbles are generated. After standing, the solution is filtered, and finally washed with deionized water until neutral. The solution is then dried to obtain graphite oxide.

[0120] (4) Graphene oxide is prepared directly by dispersing the graphene oxide obtained in step (3) into an ethanol solution and then using a 90°C high-temperature spray pyrolysis device. The inlet temperature is set to 90°C, the liquid spray rate is 50 ml / h, and the atmosphere is air. The SEM image of the product obtained by this process is shown below. Figure 11 As shown in Comparative Example 3, the reduction reaction was incomplete under ultrasound and low temperature, and the structure remained layered.

[0121] Comparative Example 4

[0122] (1) Cut the negative electrode sheet of the waste lithium-ion battery to 1cm. 2 The graphite was mixed with triethyl phosphate at a solid-liquid ratio of 1:10 g / mL. The mixture was magnetically stirred and reacted at 20°C for 60 min. After filtration, the recovered graphite was obtained with a recovery rate of 95%.

[0123] (2) The recovered graphite obtained in step (1) was vacuum dried and then leached with citric acid to remove the metal impurities, resulting in high-purity graphite. The citric acid concentration was 0.05 mol / L, the solid-liquid ratio was 1:50 g / mL, the temperature was 90℃, and the leaching time was 40 min. The removal rate of metal impurities reached 99.5%.

[0124] (3) The high-purity graphite described in step (2) is chemically oxidized using an oxidizing agent composed of concentrated sulfuric acid and potassium permanganate to form graphite oxide, with the ratio of high-purity graphite:concentrated sulfuric acid:potassium permanganate = 1g:25mL:3g. The oxidation reaction is first carried out in an ice-water bath for 90 minutes, and then at a medium temperature of 50℃ for 8 hours to obtain a graphite oxide solution. Hydrogen peroxide is then added until no more bubbles are generated. After standing, the solution is filtered, and finally washed with deionized water until neutral, and then dried to obtain graphite oxide.

[0125] (4) The graphene oxide obtained in step (3) was ultrasonically dispersed in water at 240W for 30 min and then placed in a tube furnace at 300℃ to prepare graphene. Its Raman spectrum is as follows. Figure 12As shown, the Raman test results indicate that the G peak appears at 1582.2 cm⁻¹. -1 At that point, by formula W G =1581.6+11 / (1+n) 1.6 The number of graphene layers, n, was calculated to be 5.94, approximately 6 layers.

[0126] In this comparative example, the high-temperature pyrolysis spray equipment of Example 1 was replaced with a tube furnace with the same temperature and atmosphere. The experiment found that near-monolayer graphene could not be obtained, and the number of graphene layers was 6. This indicates that the spray pyrolysis process may be more conducive to the exfoliation of graphene oxide than direct tube furnace pyrolysis.

[0127] Comparative Example 5

[0128] (1) Cut the negative electrode sheet of the waste lithium-ion battery to 1cm. 2 The graphite was mixed with triethyl phosphate at a solid-liquid ratio of 1:10 g / mL. The mixture was magnetically stirred and reacted at 20°C for 60 min. After filtration, the recovered graphite was obtained with a recovery rate of 95%.

[0129] (2) The recovered graphite obtained in step (1) was vacuum dried and then leached with citric acid to remove the metal impurities, resulting in high-purity graphite. The citric acid concentration was 0.05 mol / L, the solid-liquid ratio was 1:50 g / mL, the temperature was 90℃, and the leaching time was 40 min. The removal rate of metal impurities reached 99.5%.

[0130] (3) The high-purity graphite described in step (2) is chemically oxidized using an oxidizing agent composed of concentrated sulfuric acid and potassium permanganate to form graphite oxide, with the ratio of high-purity graphite:concentrated sulfuric acid:potassium permanganate = 1g:25mL:3g. The oxidation reaction is first carried out in an ice-water bath for 90 minutes, and then at a medium temperature of 50℃ for 8 hours to obtain a graphite oxide solution. Hydrogen peroxide is then added until no more bubbles are generated. After standing, the solution is filtered, and finally washed with deionized water until neutral, and then dried to obtain graphite oxide.

[0131] (4) The graphene oxide obtained in step (3) was ultrasonically dispersed in ammonia water at 240W for 30 min and then placed in a tube furnace at 300℃ to prepare graphene. Its Raman spectrum is as follows. Figure 13 As shown, the Raman test results indicate that the G peak appears at 1582.8 cm⁻¹. -1 At that point, by formula W G =1581.6+11 / (1+1.6n) calculates the number of graphene layers n to be 3.71, which is approximately 4 layers.

[0132] The results of Comparative Example 5 show that the graphene prepared by dispersing and then pyrolyzing with ammonia water has a closer number of layers than that prepared by directly using a tube furnace in Comparative Example 4. This may be due to the combined effect of chemical and thermal reduction. However, it is still not as good as the high-temperature spray pyrolysis in the examples, where direct high-temperature spray pyrolysis can prepare near-monolayer graphene.

[0133] Comparative Example 6

[0134] (1) Purchased flake graphite was chemically oxidized using an oxidizing agent composed of concentrated sulfuric acid and potassium permanganate to form graphite oxide. The ratio of flake graphite: concentrated sulfuric acid: potassium permanganate was 1g: 25mL: 3g. The oxidation reaction was first carried out in an ice-water bath for 90 minutes, and then at a medium temperature of 50℃ for 8 hours to obtain a graphite oxide solution. Hydrogen peroxide was then added until no more bubbles were generated. After standing, the solution was filtered, and finally washed with deionized water until neutral. The solution was then dried to obtain graphite oxide.

[0135] (2) The graphene oxide obtained in step (1) was ultrasonically dispersed in water at 240W for 30 minutes, and then graphene was prepared by high-temperature spray pyrolysis at 300℃. The air inlet temperature was 300℃, the liquid spraying rate was 50ml / h, and the atmosphere was air. The Raman spectrum of the obtained graphene is shown below. Figure 15 As shown, the Raman test results indicate that the G peak appears at 1581.6 cm⁻¹. -1 At that point, by formula W G =1581.6+11 / (1+n) 1.6 The number of graphene layers, n, was calculated to be 33.75, approximately 34 layers.

[0136] This comparative example directly used commercially available flake graphite instead of the high-purity graphite separated from the lithium-ion battery anode material in Example 1. Graphene was prepared by oxidation, ultrasonication, and spray pyrolysis. However, experiments showed that commercially available flake graphite could not yield few-layer graphene using the same method as in Example 1. The FTIR spectrum of graphite oxide obtained after the oxidation of flake graphite in step (1) was analyzed, as shown below. Figure 14 As shown, it can be seen that it is similar to Figure 4(FTIR spectrum of graphite oxide in Example 1) After comparison, it was found that graphite oxide prepared using flake graphite as raw material contained less oxygen functional groups than graphite separated from waste lithium-ion battery anode material under the same oxidation conditions. Moreover, considering that near-monolayer graphene can also be prepared using expanded graphite as raw material in Example 4, it is possible that the graphite separated from lithium-ion battery anode material increases the interlayer spacing during battery charging and discharging, achieving an interlayer expansion effect similar to expanded graphite. This makes it more conducive to the intercalation reaction of concentrated sulfuric acid, so as to incorporate more oxygen-containing functional groups, which is beneficial to the subsequent ultrasonic exfoliation and spray pyrolysis to prepare few-layer graphene. Flake graphite may not have undergone expansion itself. Judging from the measured number of graphene layers, it is not easy to obtain near-monolayer graphene according to the method of the present invention.

[0137] Comparative Example 7

[0138] (1) Cut the negative electrode sheet of the waste lithium-ion battery to 1cm. 2 The graphite was mixed with triethyl phosphate at a solid-liquid ratio of 1:10 g / mL. The mixture was magnetically stirred and reacted at 20°C for 60 min. After filtration, the recovered graphite was obtained with a recovery rate of 95%.

[0139] (2) The recovered graphite obtained in step (1) was vacuum dried and then leached with citric acid to remove the metal impurities, resulting in high-purity graphite. The citric acid concentration was 0.05 mol / L, the solid-liquid ratio was 1:50 g / mL, the temperature was 90℃, and the leaching time was 40 min. The removal rate of metal impurities reached 99.5%.

[0140] (3) The high-purity graphite described in step (2) was first reacted with concentrated sulfuric acid at a ratio of 1g:25mL, and then reacted in an ice-water bath for 30 minutes. 3g of potassium permanganate was then added. The oxidation reaction was first carried out in an ice-water bath for 90 minutes, then at a medium temperature of 50℃ for 8 hours to obtain an oxidized graphite solution. Hydrogen peroxide was then added until no more bubbles were generated. After standing, the solution was filtered, washed with deionized water until neutral, and then dried to obtain oxidized graphite.

[0141] (4) The graphene oxide obtained in step (3) was ultrasonically dispersed in water at 240W for 30 min and then subjected to high-temperature spray pyrolysis at 300℃ to prepare graphene. Its Raman spectrum is as follows: Figure 17 As shown, the Raman test results indicate that the G peak appears at 1581.7 cm⁻¹. -1 At that point, by formula W G =1581.6+11 / (1+n) 1.6 The number of graphene layers, n, was calculated to be 16.56, approximately 17 layers.

[0142] The conditions for this comparative example are the same as in Example 1, except that in step (3) of the oxidation step, the oxidizing agents concentrated sulfuric acid and potassium permanganate are added sequentially in steps, instead of being added simultaneously in Example 1. The resulting graphene has 17 layers, and few-layer graphene cannot be obtained. The FTIR spectrum of the graphene oxide obtained in step (3) is analyzed as follows... Figure 16 As shown, a comparison of the FTIR spectra of graphite oxide with those of Example 1 revealed that the graphite oxide prepared by simultaneously adding concentrated sulfuric acid and potassium permanganate to oxidize graphite had a better degree of oxidation than that prepared by adding them in steps. This provides a good foundation for the subsequent preparation of monolayer or few-layer graphene.

[0143] Comparative Example 8

[0144] (1) Cut the negative electrode sheet of the waste lithium-ion battery to 1cm. 2 The graphite was mixed with triethyl phosphate at a solid-liquid ratio of 1:10 g / mL. The mixture was magnetically stirred and reacted at 20°C for 60 min. After filtration, the recovered graphite was obtained with a recovery rate of 95%.

[0145] (2) The recovered graphite obtained in step (1) was vacuum dried and then leached with citric acid to remove the metal impurities, resulting in high-purity graphite. The citric acid concentration was 0.05 mol / L, the solid-liquid ratio was 1:50 g / mL, the temperature was 90℃, and the leaching time was 40 min. The removal rate of metal impurities reached 99.5%.

[0146] (3) The high-purity graphite described in step (2) is first added with concentrated sulfuric acid at a ratio of high-purity graphite: concentrated sulfuric acid = 1g: 25mL. After reacting in an ice-water bath for 30min, 3g of potassium permanganate is added. Then, the reaction is continued in an ice-water bath for 60min. Then, the reaction is carried out at a medium temperature of 35℃ for 0.5h. Finally, the reaction is carried out at a high temperature of 95℃ until no bubbles are generated when hydrogen peroxide is added. After standing, the mixture is filtered. Finally, it is washed with deionized water until neutral and then dried to obtain graphite oxide.

[0147] (4) The graphene oxide obtained in step (3) was ultrasonically dispersed in water at 240W for 30 min and then subjected to high-temperature spray pyrolysis at 300℃ to prepare graphene. Its Raman spectrum is as follows: Figure 18 As shown, the Raman test results indicate that the G peak appears at 1582.1 cm⁻¹. -1 At that point, by formula W G =1581.6+11 / (1+n) 1.6 The number of graphene layers, n, was calculated to be 6.71, approximately 7 layers.

[0148] Comparative Example 9

[0149] (1) Purchased flake graphite was first mixed with concentrated sulfuric acid at a ratio of graphite:concentrated sulfuric acid = 1g:25mL. After reacting in an ice-water bath for 30min, 3g of potassium permanganate was added. The oxidation reaction was first carried out in an ice-water bath for 90min, then at a medium temperature of 35℃ for 0.5h, and finally at a high temperature of 95℃ until no bubbles were produced after adding hydrogen peroxide. After standing, the mixture was filtered, washed with deionized water until neutral, and then dried to obtain graphite oxide.

[0150] (4) The graphene oxide obtained in step (3) was ultrasonically dispersed in water at 240W for 30 min and then subjected to high-temperature spray pyrolysis at 300℃ to prepare graphene. Its Raman spectrum is as follows: Figure 19 As shown, the Raman test results indicate that the G peak appears at 1581.8 cm⁻¹. -1 At that point, by formula W G =1581.6+11 / (1+n) 1.6 The number of graphene layers, n, was calculated to be 12.09, approximately 12 layers.

[0151] Comparative Example 10

[0152] (1) Purchased expanded graphite was first mixed with concentrated sulfuric acid at a ratio of graphite:concentrated sulfuric acid = 1g:25mL. After reacting in an ice-water bath for 30min, 3g of potassium permanganate was added. The oxidation reaction was first carried out in an ice-water bath for 90min, then at a medium temperature of 35℃ for 0.5h, and finally at a high temperature of 95℃ until no bubbles were produced after adding hydrogen peroxide. After standing, the mixture was filtered, washed with deionized water until neutral, and then dried to obtain graphite oxide.

[0153] (4) The graphene oxide obtained in step (3) was ultrasonically dispersed in water at 240W for 30 min and then subjected to high-temperature spray pyrolysis at 300℃ to prepare graphene. Its Raman spectrum is as follows: Figure 20 As shown in the figure, the Raman test results indicate that the G peak appears at 1582.0 cm⁻¹. -1 At that point, by formula W G =1581.6+11 / (1+n) 1.6 The number of graphene layers, n, was calculated to be 7.75, approximately 8 layers.

[0154] Comparative Examples 8, 9, and 10 respectively used recycled graphite, commercially available flake graphite, and commercially available expanded graphite separated from lithium-ion battery anodes. They were oxidized using a low-temperature-medium-high-temperature oxidation process, with concentrated sulfuric acid and potassium permanganate added separately as oxidants. Other steps were the same as in Example 1. The experimental results showed that near-monolayer graphene could not be obtained. The recycled graphite and expanded graphite obtained graphene with 7 and 8 layers respectively, while the flake graphite could only obtain graphene with 12 layers. This further demonstrates that the recycled graphite and expanded graphite separated from lithium-ion battery anode materials do indeed have similar expanded layered structures.

[0155] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing single-layer or few-layer graphene using expanded graphite as raw material, characterized in that, Includes the following steps: (1) Expanded graphite is oxidized with an oxidizing agent to form graphite oxide; the expanded graphite is expanded graphite and / or recycled graphite separated from lithium-ion battery negative electrode material; the oxidizing agent is a mixture of concentrated sulfuric acid and potassium permanganate; the oxidation is specifically as follows: the expanded graphite is mixed with the mixed oxidizing agent of concentrated sulfuric acid and potassium permanganate and reacted at an ice-water bath temperature for 60-180 min, and then the temperature is raised to a medium temperature of 30-70 °C for 4-10 h, without a high-temperature reaction step, to obtain graphite oxide; (2) The graphene oxide is ultrasonically dispersed in a solvent and then prepared by high-temperature spray pyrolysis to obtain single-layer or few-layer graphene with less than or equal to 2 layers.

2. The method as described in claim 1, characterized in that, The process of obtaining the recycled graphite includes the following steps: separating the disassembled lithium-ion battery anode material by soaking it in an organic solvent to obtain waste graphite; and then washing the waste graphite with acid to remove impurities and drying it to obtain the purified recycled graphite, which is the recycled graphite separated from the lithium-ion battery anode material.

3. The method as described in claim 1, characterized in that, The lithium-ion battery is one or more of the following lithium-ion batteries with lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium iron phosphate as the positive electrode material.

4. The method as described in claim 2, characterized in that, The organic solvent is one or more selected from methanol, ethanol, isopropanol, acetone, N-methylformamide, N-methylpyrrolidone, dimethylacetamide, triethyl phosphate, and ionic liquids; and / or, The solid-liquid ratio of the disassembled lithium-ion battery negative electrode material to the organic solvent is 1:10-1:100 g / mL, the immersion temperature is 20-60 ℃, and the time is 5-60 min; and / or, The impurity removal reagent used in the acid washing is Lewis acid, and the concentration of Lewis acid is 0.05-3 mol / L; the solid-liquid ratio of waste graphite to Lewis acid is 1:10-1:200 g / mL, the reaction time is 10-90 min, and the temperature is 30-90 ℃.

5. The method as described in claim 1, characterized in that, The oxidation in step (1) is as follows: the expanded graphite is mixed with a mixture of concentrated sulfuric acid and potassium permanganate as an oxidant and then reacted at an ice-water bath temperature for 60-90 min, and then the temperature is raised to a medium temperature of 40-60°C for 7-9 h.

6. The method as described in claim 5, characterized in that, The ratio of expanded graphite, concentrated sulfuric acid, and potassium permanganate used is 1 g:(15-25) mL:(1-5) g.

7. The method as described in claim 1, characterized in that, The ultrasonic dispersion power in step (2) is 150 W-270 W; the ultrasonic dispersion time is 10-60 min.

8. The method as described in claim 1, characterized in that, The solvent in step (2) is one or more of water, isopropanol, N-methylformamide, methanol and ethanol.

9. The method as described in claim 1, characterized in that, The high-temperature spray pyrolysis described in step (2) has a pyrolysis temperature of 100-500 ℃.

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