Method for recycling graphene from waste batteries and graphene

Abstract

This application provides a method for recycling and regenerating graphene from waste batteries and the resulting graphene. This method uses a mixture of acid and hydrogen peroxide to treat the recovered graphite, effectively leaching metallic impurities from the graphite and slightly oxidizing it. This improves the graphite purity and increases the interlayer spacing, significantly enhancing the exfoliation yield and the quality of the resulting graphene.

Description

Technical Field

[0001] The present application relates to the technical field of recycling of waste batteries, and in particular to a method for recycling and regenerating graphene from waste batteries and the graphene. Background Art

[0002] Graphite has become an irreplaceable negative electrode active material for commercial lithium batteries due to its outstanding physical and chemical properties and structural advantages. Currently, with the large-scale application of lithium batteries, the number of scrapped batteries is also increasing year by year, which leads to serious waste of resources and environmental pollution. In addition, the price of negative electrode graphite in the market is relatively high, while the negative electrode graphite in waste lithium batteries still has a good intrinsic structure. Therefore, the recycling and reuse of waste lithium batteries, especially the recycling and reuse of graphite materials in batteries, has high environmental and commercial value. However, there are currently few literature reports on the preparation of high-value graphene from recycled negative electrode graphite, and most of the processes are cumbersome and the product consistency is poor, making them difficult to industrialize. Summary of the Invention

[0003] In light of this, the present application provides a method for recycling graphene from waste batteries and the resulting graphene. This method utilizes a mixture of acid and hydrogen peroxide to treat the recovered graphite, simultaneously achieving efficient leaching of metallic impurities from the graphite and mild oxidation of the graphite, thereby increasing the graphite purity and the interlayer spacing, significantly improving the graphite exfoliation yield and the quality of the resulting graphene.

[0004] Specifically, the first aspect of the present application provides a method for recycling graphene from waste batteries, comprising the following steps:

[0005] (1) discharging, crushing, magnetic separation and cyclone separation of waste batteries to separate a mixture containing graphite, then acid leaching the mixture to obtain a solid, drying and calcining the solid to obtain graphite slag containing metal impurities;

[0006] (2) adding the graphite slag to a mixture of an acid solution and a hydrogen peroxide solution to leach out metal impurities in the graphite slag, slightly oxidize the graphite, and introduce intercalants between the graphite layers to obtain a first dispersion; wherein the intercalants include at least one of acid molecules and water molecules;

[0007] (3) subjecting the first dispersion to a solid-liquid separation treatment, collecting a first precipitate and adding it to a stripping agent, adjusting the pH value of the system, and performing a stripping treatment, so that the graphite is stripped into graphene under the combined action of the stripping agent and the intercalant, thereby obtaining a second dispersion containing graphene;

[0008] (4) subjecting the second dispersion containing graphene to solid-liquid separation, collecting the upper layer of liquid for secondary solid-liquid separation, and collecting a second precipitate; washing and drying the second precipitate to obtain graphene powder.

[0009] Optionally, the solid-liquid mass volume ratio of the graphite slag to the acid solution is 1:(5-200) g / mL.

[0010] Optionally, the solid-liquid mass volume ratio of the graphite slag to the hydrogen peroxide solution is 1:(0.1-20) g / mL.

[0011] Optionally, in step (2), the mass ratio of the solute acid in the acid solution to the hydrogen peroxide solute in the hydrogen peroxide solution is 1:(0.001-0.5).

[0012] Optionally, the acid includes at least one of sulfuric acid, nitric acid, hydrochloric acid, and perchloric acid.

[0013] Optionally, in step (2), the graphite slag is added to a mixture of an acid solution and a hydrogen peroxide solution and mixed, and the mixing includes any one of ultrasonic treatment, magnetic stirring, mechanical stirring and homogeneous mixing.

[0014] Optionally, in step (2), the graphite slag is added to a mixture of an acid solution and a hydrogen peroxide solution and mixed, the mixing time is 0.5h-24h; the temperature is 0°C-95°C, preferably 40°C-80°C.

[0015] Optionally, the mass volume ratio of the first precipitate to the stripping agent is 1 mg / mL-300 mg / mL.

[0016] Optionally, the solid-liquid separation treatment in step (3) and the secondary solid-liquid separation in step (4) include either centrifugal separation or vacuum filtration. Preferably, the rotation speed of the centrifugal separation is 2000 rpm / min-10000 rpm / min; and the vacuum degree of the vacuum filtration is 0.85 MPa-0.95 MPa.

[0017] Optionally, the stripping agent comprises any one of pure water, an aqueous solution of a surfactant, and an organic solvent, preferably an aqueous solution of a surfactant.

[0018] Optionally, the surfactant includes at least one of polyvinyl pyrrolidone, polyethylene glycol, polyvinyl alcohol, Tween 80, sodium cholate, sodium dodecyl sulfonate, sodium linear alkylbenzene sulfonate and sodium α-olefin sulfonate.

[0019] Optionally, the mass percentage of the surfactant in the surfactant aqueous solution is 0.01-5%.

[0020] Optionally, the organic solvent includes at least one of N-methylpyrrolidone, ethanol, dimethylformamide and dimethyl sulfoxide.

[0021] Optionally, in step (3), the pH value of the system is adjusted to 7-11.

[0022] Optionally, the delamination treatment in step (3) is at least one of ultrasonic delamination, thermal delamination, grinding delamination and homogeneous delamination.

[0023] Optionally, the peeling treatment takes 0.5h-48h.

[0024] Optionally, in step (4), the solid-liquid separation comprises centrifugation or natural sedimentation. Preferably, the centrifugation speed is 2000 rpm / min-10000 rpm / min, and the time is 1 min-20 min. Preferably, the natural sedimentation time is 2 h-24 h.

[0025] Preferably, the washing in step (4) comprises repeated washing with deionized water and anhydrous ethanol.

[0026] Preferably, the drying in step (4) includes any one of normal pressure drying, vacuum drying, spray drying, flash drying and freeze drying.

[0027] Optionally, step (4) further includes heat treating the graphene powder.

[0028] The second aspect of the present application provides a graphene, wherein the defect characteristic peak of the Raman spectrum of the graphene is sp 2 The ratio of the intensity values ​​of the in-plane vibration peaks of the carbon atoms is 0.2-0.4, and the electrical conductivity of the graphene is not less than 65000 S / m.

[0029] Optionally, the number of graphene layers is 1-10.

[0030] Optionally, the graphene is prepared according to the method provided in the first aspect of this application.

[0031] The method of the present application can efficiently prepare high-quality graphene from graphite in waste batteries. The prepared graphene has low impurity content, narrow layer distribution, low defect density, and high electrical conductivity. It can be used as an efficient conductive agent for lithium batteries to improve the capacity and rate performance of lithium batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Scanning electron microscope (SEM) photos of the waste graphite slag raw materials used in each embodiment and comparative example of the present application;

[0033] Figure 2 This is an SEM photograph of the first precipitate prepared in Comparative Example 1;

[0034] Figure 3 This is a SEM photo of graphene prepared in Comparative Example 1;

[0035] Figure 4 This is a transmission electron microscope (TEM) photograph of the graphene prepared in Comparative Example 1;

[0036] Figure 5 This is a SEM photo of the first precipitate prepared in Example 1;

[0037] Figure 6 This is a SEM photo of the graphene obtained in Example 1;

[0038] Figure 7 TEM image of graphene obtained in Example 1;

[0039] Figure 8 This is a SEM photo of graphene prepared in Comparative Example 2;

[0040] Figure 9 TEM image of graphene prepared in Comparative Example 2;

[0041] Figure 10 X-ray diffraction (XRD) spectra of the graphite slag (a), the first precipitate (b), and the graphene powder (c) obtained in Example 1;

[0042] Figure 11 The graphite slag, the graphene prepared in Example 1, and the graphene prepared in Comparative Example 1 and Comparative Example 2 are Raman spectra (Raman spectra, Raman) curves. DETAILED DESCRIPTION

[0043] The technical solutions of the embodiments of the present application are described in detail below.

[0044] The present invention provides a method for recycling graphene from waste batteries, comprising the following steps:

[0045] (1) discharging, crushing, magnetic separation and cyclone separation of waste batteries to separate a mixture containing graphite, then acid leaching the mixture to obtain a solid, drying and calcining the solid to obtain graphite slag containing metal impurities;

[0046] (2) adding the graphite slag to a mixture of an acid solution and a hydrogen peroxide solution to leach out metal impurities in the graphite slag, slightly oxidize the graphite, and introduce intercalants between the graphite layers to obtain a first dispersion; wherein the intercalants include at least one of acid molecules and water molecules;

[0047] (3) subjecting the first dispersion to a solid-liquid separation treatment, collecting a first precipitate and adding it to a stripping agent, adjusting the pH value of the system, and performing a stripping treatment, so that the graphite is stripped into graphene under the combined action of the stripping agent and the intercalant, thereby obtaining a second dispersion containing graphene;

[0048] (4) subjecting the second dispersion containing graphene to solid-liquid separation, collecting the upper layer of liquid for secondary solid-liquid separation, and collecting a second precipitate; washing and drying the second precipitate to obtain graphene powder.

[0049] Waste batteries contain solid components such as separators and positive and negative electrode plates. These plates contain electrode active materials, binders, and other substances. The waste batteries are first pre-treated to produce graphite slag: the batteries are discharged and crushed, and the crushed material is subjected to magnetic separation and cyclone separation to remove most of the iron, copper, and aluminum, resulting in a mixture containing the positive electrode active material and negative electrode graphite. Since the positive electrode active material contains a large amount of metallic impurities, this mixture is also subjected to acid leaching to remove most of the metallic impurities, and then a solid product is separated. This solid product is then dried and calcined to remove organic matter such as the binder and separator, resulting in graphite slag containing metallic impurities.

[0050] The present application uses a mixed solution of acid and hydrogen peroxide to treat graphite slag, and hydrogen peroxide, under the synergistic effect of acid, can slightly oxidize the above-mentioned graphite, that is, without introducing additional strong oxidants, oxygen-containing functional groups (such as hydroxyl, carboxyl, etc.) can be covalently bonded on the graphite layer, and acid molecules and water molecules are promoted to be inserted into the interlayer of graphite. The oxygen-containing functional groups on the above-mentioned graphite layer and the intercalation material between the graphite layers can effectively weaken the van der Waals force between the graphite layers, so that the interlayer spacing of graphite is increased, thereby significantly reducing the difficulty of peeling of graphite. The above-mentioned oxidation is relatively weak to the destructive power of the graphite sheet structure, which can significantly reduce the structural defects of the graphene obtained, and then the yield rate of graphene after peeling and the uniformity of graphene quality can be improved.

[0051] During the stripping process, water molecules, acid molecules between the graphite layers, and most of the oxygen-containing functional groups on the graphite layers can be detached under the synergistic action of the stripping agent and the force field, thereby obtaining a second dispersion containing graphene. The second dispersion is then subjected to a washing treatment in step (4) to further remove impurities and possible residual intercalants and oxygen-containing functional groups in the second precipitate. In particular, the interaction between oxygen-containing functional groups and graphite sheets is stronger than that between water molecules and acid molecules. Therefore, in the graphene obtained after washing, there are a few sheets with individual oxygen-containing functional groups attached. The presence of these very few oxygen-containing functional groups can improve the dispersibility of the graphene in the solvent, which in turn is beneficial for the application of the above-mentioned graphene as a conductive agent in lithium batteries.

[0052] In addition, the combination of acid and hydrogen peroxide can remove the metal impurities that are not completely removed in step (1) and still remain in the graphite slag. In particular, hydrogen peroxide can reduce the high-valent metal ions remaining in the graphite slag to more easily soluble low-valent metal ions (for example, reducing the high-valent cobalt ions and manganese ions in the graphite slag to Co 2+ 、Mn 2+ ) and, in conjunction with an acid, leaching the aforementioned low-valent metal ions from the graphite slag to improve the purity of the resulting graphene. Therefore, this method can efficiently convert graphite from waste batteries into graphene with low impurity content, a narrow layer number distribution, low defect density, and high electrical conductivity.

[0053] In the embodiment of the present application, the conditions of discharge, crushing, magnetic separation and cyclone separation, acid leaching and calcination in step (1) can be set by those skilled in the art according to the system and state of the waste batteries used, and will not be described in detail here.

[0054] In an embodiment of the present application, the solid-to-liquid mass-to-volume ratio of the graphite slag to the acid solution (i.e., the mass of the graphite slag to the volume of the acid solution) is 1:(5-200) g / mL. For example, it can be 1:10, 1:20, 1:30, 1:40, 1:50, 1:100, 1:150, 1:200, etc. A suitable solid-to-liquid ratio facilitates the removal of metallic impurities from the graphite slag, and an appropriate amount of acid can provide a higher oxidation potential, which facilitates the insertion of the intercalant into the interlayers of the graphite.

[0055] In an embodiment of the present application, the solid-to-liquid mass-to-volume ratio of the graphite slag to the hydrogen peroxide solution (i.e., the mass of the graphite slag to the volume of the hydrogen peroxide solution) is 1:(0.1-20) g / mL. For example, it can be 1:5, 1:10, 1:15, 1:20, etc. A suitable solid-to-liquid ratio between the graphite slag and the hydrogen peroxide solution helps ensure the oxidative properties of the system, i.e., it can increase the content of oxygen-containing functional groups in the graphite and also promote the intercalation of water molecules and acid molecules between the graphite layers.

[0056] In an embodiment of the present application, the acid comprises at least one of nitric acid, sulfuric acid, perchloric acid, and hydrochloric acid. The acid solution used in the present application is a diluted or undiluted commercially available acid solution, with an appropriate concentration to maintain the oxidation potential of the system. The mass fraction of hydrochloric acid can be 10%-30%, the mass fraction of nitric acid or perchloric acid can be 5%-50%, and the mass fraction of sulfuric acid can be 10%-98%. Preferably, the mass fraction of nitric acid or perchloric acid can be 20%-50%, and the mass fraction of sulfuric acid can be 50%-98%.

[0057] In this application, the hydrogen peroxide solution used is a commercially available hydrogen peroxide solution.

[0058] In the embodiment of the present application, in step (2), the mass ratio of the solute acid in the acid solution to the hydrogen peroxide solute in the hydrogen peroxide solution is 1:(0.001-0.5). For example, the above ratio can be 1:0.05, 1:0.1, 1:0.15, 1:0.2, 1:0.25, 1:0.3, 1:0.35, 1:0.4, 1:0.45, 1:0.5, etc. By controlling the mass ratio of the solute acid in the acid solution to the hydrogen peroxide solute in the hydrogen peroxide solution within a certain range, the oxidation ability of the system on graphite can be maintained, thereby smoothly achieving weak oxidation of graphite.

[0059] In the embodiment of the present application, in step (2), the graphite slag is added to the mixture of the acid solution and the hydrogen peroxide solution and mixed, and the mixing includes any one of ultrasonic treatment, magnetic stirring, mechanical stirring, and homogeneous mixing. Preferably, ultrasonic treatment and homogeneous mixing are used. The cavitation effect of the ultrasound or the high pressure of the homogenizer is conducive to the chemical reaction between the graphite, the acid, and the hydrogen peroxide.

[0060] In the embodiment of the present application, the graphite slag is added to a mixture of an acid solution and a hydrogen peroxide solution and mixed, and the mixing time is 0.5h-24h.

[0061] In an embodiment of the present application, the graphite slag is added to a mixture of an acid solution and a hydrogen peroxide solution and mixed at a temperature of 0°C to 95°C, preferably 40°C to 80°C. A suitable temperature facilitates the leaching of metallic impurities from the graphite slag and also facilitates the oxidation of the graphite. Furthermore, a suitable temperature can mitigate the decomposition of hydrogen peroxide in the system, which can affect the oxidation effect.

[0062] In an embodiment of the present application, the solid-to-liquid ratio of the first precipitate to the stripping agent is 1 mg / mL to 300 mg / mL. For example, it can be 1 mg / mL, 5 mg / mL, 10 mg / mL, 50 mg / mL, 100 mg / mL, 200 mg / mL, 300 mg / mL, etc. A suitable solid-to-liquid ratio helps improve the dispersion of the first precipitate in the stripping agent, thereby improving stripping efficiency.

[0063] In the embodiment of the present application, the solid-liquid separation treatment in step (3) and the secondary solid-liquid separation treatment in step (4) include either centrifugal separation or vacuum filtration. Preferably, the rotation speed of the centrifugal separation is 2000 rpm / min-10000 rpm / min; the vacuum degree of the vacuum filtration is 0.85 MPa-0.95 MPa.

[0064] In an embodiment of the present application, the stripping agent comprises any one of pure water, an aqueous solution of a surfactant, and an organic solvent. Preferably, the aqueous solution of a surfactant is employed. Graphite and the resulting graphene can be better dispersed in the stripping agent. Specifically, the surfactant can reduce the surface tension of the liquid phase and, while adsorbing on the graphite surface, create a certain degree of steric hindrance, thereby promoting uniform dispersion of the graphite in the liquid phase.

[0065] In the embodiment of the present application, the mass percentage of the surfactant in the surfactant aqueous solution is 0.01-5%. The surfactant aqueous solution with a suitable concentration is conducive to the dispersion of graphite and the prepared graphene.

[0066] In an embodiment of the present application, the surfactant includes at least one of polyvinyl pyrrolidone, polyethylene glycol, polyvinyl alcohol, Tween 80, sodium cholate, sodium dodecyl sulfonate, sodium linear alkylbenzene sulfonate and sodium α-olefin sulfonate.

[0067] In an embodiment of the present application, the organic solvent includes at least one of N-methylpyrrolidone, ethanol, dimethylformamide and dimethyl sulfoxide.

[0068] In the embodiment of the present application, in step (3), the pH value is 7-11. Adjusting the pH value of the system to the above range is conducive to the exfoliation of graphite and can promote the removal of intercalants and oxygen-containing functional groups (especially oxygen-containing functional groups) between graphite layers.

[0069] In an embodiment of the present application, the delamination treatment in step (3) is at least one of ultrasonic delamination, pyrolysis delamination, grinding delamination, and homogenization delamination. Ultrasonic delamination is performed using an ultrasonic crusher, pyrolysis delamination is performed using a high-temperature device, grinding delamination is performed using a ball mill, sand mill, or colloid mill, and homogenization delamination is performed using a homogenizer. These delamination methods are beneficial for improving the delamination efficiency of graphite.

[0070] In the embodiment of the present application, the time of the delamination treatment is 0.5h-48h. Sufficient time is conducive to improving the uniformity of the quality of the prepared product.

[0071] Optionally, in step (4), the solid-liquid separation treatment includes centrifugation or natural sedimentation. Preferably, the centrifugation speed is 2000 rpm / min-10000 rpm / min, and the time is 1 min-20 min. Preferably, the natural sedimentation time is 2 h-24 h.

[0072] In the embodiment of the present application, the washing in step (4) includes repeated washing with deionized water and anhydrous ethanol. This operation is to remove the intercalated substances and some oxygen-containing functional groups in the second precipitate, i.e., water molecules, acid molecules, and some oxygen-containing functional groups. Repeated washing can ensure the purity of the resulting graphene.

[0073] In an embodiment of the present application, the drying in step (4) includes any one of normal pressure drying, vacuum drying, spray drying, flash drying and freeze drying.

[0074] In some embodiments of the present application, step (4) further comprises heat-treating the graphene powder. Further heat-treating the obtained graphene powder can completely remove the very few oxygen-containing functional groups remaining between the graphene layers, further improving the integrity of the graphene structure.

[0075] Correspondingly, the embodiment of the present application further provides a graphene, wherein the defect characteristic peak (D peak) of the Raman spectrum of the graphene is 2 The ratio of the intensity values ​​of the carbon atom in-plane vibration peak (G peak) (I D / I G ) is 0.2-0.4, and the conductivity of the graphene is not less than 65000S / m. The D peak reflects the structural defect density and disorder of the graphene; the G peak represents the sp 2 In-plane vibration mode of hybridized carbon atoms. D / I G The value is low, indicating that there are certain structural defects in its structure, but the defect density is low. Such a structure can improve the conductivity of the graphene. Compared with the graphene prepared by the redox method, the graphene prepared by the method of the present invention is D / I G The lower the value, the lower the structural defect density, and the higher the electrical conductivity. This higher electrical conductivity makes the graphene a high-quality conductive agent for batteries. Formulating the graphene into a high-concentration graphene conductive slurry for preparing battery electrodes can significantly improve the capacity and rate performance of the battery.

[0076] In some embodiments of the present application, the number of graphene layers is 1 to 10. The graphene layer number distribution is relatively narrow, close to an ideal two-dimensional material structure.

[0077] In some embodiments of the present application, the above-mentioned graphene is prepared by the method provided by the present application, and the graphene has low impurity content, narrow layer number distribution, low defect density, and high electrical conductivity.

[0078] The technical solution of this application is described in detail below with reference to specific embodiments.

[0079] Example 1

[0080] (1) Discharging, crushing, magnetic separation and cyclone separation of waste batteries to separate a mixture containing graphite, and then subjecting the mixture to H + Acid leaching was performed in an acid solution with a concentration of 2 mol / L for 4 hours, and a solid was separated and dried, and then calcined at 450° C. for 2 hours to obtain graphite slag containing metal impurities.

[0081] (2) 1 g of graphite slag was added to 10 mL of a mixed solution of nitric acid and sulfuric acid (the sum of the masses of nitric acid and sulfuric acid accounted for 50% of the mass of the mixed solution), and then 0.8 mL of a 30% hydrogen peroxide solution was added. The mixture was magnetically stirred at 25 ° C for 4 h to obtain a first dispersion.

[0082] (3) The first dispersion was centrifuged, the precipitate was collected and 30 mL of polyvinyl pyrrolidone aqueous solution (the mass percentage of the surfactant in the above solution was 2%) was added, the pH was adjusted to 7, and the homogenized layer was peeled for 3 h to obtain a second dispersion containing graphene.

[0083] (4) The second dispersion containing graphene is centrifuged at a centrifugal speed of 8000 rpm / min for 10 min, and the upper layer of liquid is collected for vacuum filtration, repeatedly washed, and spray-dried to obtain graphene powder.

[0084] (5) Battery preparation: The prepared graphene powder is used as a conductive additive and assembled into a button-type lithium-ion battery for electrical performance testing. The method for preparing a button-type lithium-ion battery is a method well known to those skilled in the art, specifically: commercial lithium iron phosphate active material, binder polyvinylidene fluoride and graphene powder prepared in the example are mixed in a mass ratio of 96:3:1, an appropriate amount of N-methylpyrrolidone is added and stirred, and then the mixture is coated on aluminum foil and dried. The dried electrode is rolled and punched into a circular electrode of appropriate size as the positive electrode, the lithium metal sheet is used as the negative electrode, and the separator is a lithium battery separator. The above-mentioned positive electrode sheet, negative electrode sheet and separator are stacked and wound in sequence to obtain a battery cell. The cell is placed in a battery casing and injected with an electrolyte, wherein the electrolyte is a 1.0 mol / L LiPF6 solution. The battery casing is sealed in a glove box to obtain a battery. The obtained battery is denoted as S1.

[0085] Example 2

[0086] The difference from Example 1 is that in step (2), 1 g of graphite slag was added to 5 mL of a mixed solution of nitric acid and sulfuric acid (the sum of the masses of the nitric acid and sulfuric acid accounted for 50% of the mass of the mixed solution), 0.1 mL of a 30% hydrogen peroxide solution was added, and the mixture was magnetically stirred at 25°C for 4 hours to obtain a first dispersion. The resulting battery is denoted as S2.

[0087] Example 3

[0088] The difference from Example 1 is that in step (2), 1 g of graphite slag was added to 200 mL of a mixed solution of nitric acid and sulfuric acid (the sum of the masses of the nitric acid and sulfuric acid accounted for 50% of the mass of the mixed solution), 20 mL of a 30% hydrogen peroxide solution was added, and magnetic stirring was performed at 25°C for 4 hours to obtain a first dispersion. The resulting battery is denoted as S3.

[0089] Example 4

[0090] The difference from Example 1 is that the mass concentration and amount of the acid solution are adjusted so that the mass ratio of the solute acid to the solute hydrogen peroxide in the mixed solution is 1:0.1. The resulting battery is denoted as S4.

[0091] Example 5

[0092] The difference from Example 1 is that the mass concentration and amount of the acid solution are adjusted so that the mass ratio of the solute acid to the solute hydrogen peroxide in the mixed solution is 1:0.2. The resulting battery is designated as S5.

[0093] Example 6

[0094] The difference from Example 1 is that the mass concentration and amount of the acid solution are adjusted so that the mass ratio of the solute acid to the solute hydrogen peroxide in the mixed solution is 1:0.5. The resulting battery is designated as S6.

[0095] Example 7

[0096] The difference from Example 1 is that in step (3), the pH value of the system is adjusted to 11. The obtained battery is denoted as S7.

[0097] Comparative Example 1

[0098] The difference from Example 1 is that in step (2), 1 g of graphite slag was added to 10 mL of a mixed solution of nitric acid and sulfuric acid (the sum of the masses of the nitric acid and sulfuric acid accounted for 50% of the mass of the mixed solution), hydrogen peroxide was not added, and magnetic stirring was performed at 25°C for 4 hours to obtain a first dispersion. The resulting battery is denoted as DS1.

[0099] Comparative Example 2

[0100] (1) Discharging, crushing, magnetic separation and cyclone separation of waste batteries to separate a mixture containing graphite, and then subjecting the mixture to H + Acid leaching was performed in an acid solution with a concentration of 2 mol / L for 4 hours, and a solid was separated and dried, and then calcined at 450° C. for 2 hours to obtain graphite slag containing metal impurities.

[0101] (2) 1 g of graphite slag was added to 10 mL of a mixed solution of nitric acid and sulfuric acid (the sum of the masses of the nitric acid and sulfuric acid accounted for 50% of the mass of the mixed solution), without adding hydrogen peroxide, and magnetically stirred at 25° C. for 4 h to obtain a first dispersion.

[0102] (3) The first dispersion was centrifuged, and the collected precipitate was oxidized using the improved Hummers method. The specific steps were as follows: adding 30 mL of 98% concentrated sulfuric acid, slowly adding 3 g of potassium permanganate at 0°C-10°C, heating to 35°C for 2 h, then diluting with 300 mL of water and adding 3 mL of hydrogen peroxide, followed by repeated water washing and adjusting the pH to about 7, homogenizing and peeling for 3 h, and adding a reducing agent for reduction to obtain a second dispersion containing graphene.

[0103] (4) The second dispersion containing graphene is centrifuged at a centrifugal speed of 8000 rpm / min for 10 min, and the upper layer of liquid is collected for vacuum filtration, washing, and spray drying to obtain graphene powder.

[0104] (5) A button cell was prepared according to the method described in Example 1. The prepared cell was designated as DS2.

[0105] Comparative Example 3

[0106] Comparative Example 3 uses a commercial carbon black conductive agent with a mass fraction of 1% as the conductive agent of the battery to replace the graphene prepared in each Example and Comparative Examples 1-2. A button cell is prepared according to the method described in Example 1, and the prepared battery is recorded as DS3.

[0107] The graphene obtained in the above examples and comparative examples was characterized for morphology and structure, and tested for electrical conductivity. The specific testing methods are as follows:

[0108] (1) Inductively coupled plasma (ICP) emission spectrometer (ICP) testing: A certain amount of graphene powder was weighed and added to a digestion tank. An appropriate amount of aqua regia was then added for microwave digestion. After digestion, the trace element content in the digestion solution was measured using an ICP emission spectrometer. The results are summarized in Table 1.

[0109] (2) X-ray diffraction test

[0110] An appropriate amount of sample was prepared and placed in an XRD chamber for XRD scanning. The test conditions were: Cu target (λ = 0.15418 nm), Ni filter, tube voltage set to 40 kV, scan rate 5° / min, and scan angle range 5°-90°. The results are summarized in Table 2.

[0111] (3) Raman spectroscopy test

[0112] Take an appropriate amount of sample and place it on a glass slide. Use a micro-laser Raman spectrometer with a laser wavelength of 514nm to measure the Raman spectra of graphite slag and graphene powder. -1 The defect characteristic peak (D peak) of graphene around 1580cm -1 Left and right sp 2 The defect density of graphene was characterized by the intensity ratio of the carbon atom's in-plane vibration peak (G peak). The results are summarized in Table 2.

[0113] (4) Determination of the number of graphene layers

[0114] The thickness of the graphene was measured using atomic force microscopy (AFM), and the number of graphene layers was approximately calculated based on the theoretical thickness of a single-layer graphene (0.335 nm). The results are summarized in Table 2.

[0115] (5) Micromorphology test

[0116] Scanning electron microscopy was used to characterize the surface morphology of graphite slag, the first precipitate and graphene powder samples; transmission electron microscopy was used to observe the micromorphology of graphene powder.

[0117] (6) Graphene conductivity test

[0118] An appropriate amount of graphene powder was placed in a mold, the sample was compacted with a pressure of 20 MPa, and the resistivity of the sample was tested using a four-probe tester, where conductivity = 1 / resistivity.

[0119] (7) Graphene yield test

[0120] Graphene yield = (graphene powder mass / graphite slag mass) × 100%. The results are summarized in Table 2.

[0121] The batteries prepared in the above examples and comparative examples were subjected to charge and discharge tests. Specifically, the batteries were charged and discharged at 0.1C on a LAND CT2001C secondary battery performance tester at 25±1°C. The steps were as follows: 10 minutes of idling; 0.1C constant current charging to 3.8V, constant voltage charging to 0.05C cutoff; 10 minutes of idling; 0.1C constant current discharging to 2.0V, which was the first cycle. The first charge specific capacity and first discharge specific capacity were then calculated. First coulombic efficiency = (first charge specific capacity / first discharge specific capacity) * 100%.

[0122] The battery was charged and discharged at 2C at 25±1°C. The steps were as follows: 10 minutes of idling; 2C constant current charging to 3.8V, constant voltage charging to 0.05C cutoff; 10 minutes of idling; 2C constant current discharge to 2.0V, representing the 2C rate cycle, from which the 2C rate discharge specific capacity was calculated. Five batteries were selected for each example and comparative example for testing, and the average of the five battery results for each group was taken. The test results for each battery are summarized in Table 3.

[0123] Table 1 Content of metal elements in graphene and graphite slag obtained in Examples and Comparative Examples

[0124]

[0125]

[0126] Table 2 Summary of the physical and chemical indexes of graphene obtained in the examples and comparative examples

[0127] Sample number Graphene yield / % Number of graphene layers <![CDATA[I D / I G value]]> Conductivity (S / m) Example 1 75 1-10 0.28 65568 Example 2 71 1-10 0.25 69880 Example 3 73 1-10 0.36 65122 Example 4 76 1-10 0.28 68023 Example 5 80 1-10 0.26 66372 Example 6 79 1-10 0.31 65387 Example 7 75 1-10 0.27 67786 Comparative Example 1 10 5-25 0.16 35674 Comparative Example 2 85 1-10 1.20 41169

[0128] Table 3 Electrochemical performance test results of batteries prepared in Examples and Comparative Examples

[0129]

[0130]

[0131] As can be seen from the data in Table 1, the graphite slag raw material used in the present application contains a variety of metallic impurities, and a total content of about 2%, and the content of metallic impurities in the Graphene obtained after processing through the embodiments and comparative examples has dropped to a very low level, and the Graphene obtained by the embodiment has a lower impurity content and a higher purity than that of the comparative example. This illustrates that the efficient purification of graphite slag can be achieved using the method provided by the application, which is mainly due to the synergistic effect of hydrogen peroxide and acid. In addition, the method provided by the application does not contain the introduction of metallic impurities (for example, strong oxidants such as potassium permanganate) during the preparation process of graphene, and is also conducive to further reducing the content of metallic impurities in the product, improving the purity of the product.

[0132] The morphology of each sample is analyzed below. Figures 1-10 . Figure 1 The graphite slag obtained by the method provided in Comparative Example 1 (i.e., the first precipitate, see Figure 2 ) is still a compact crystalline structure, the graphite slag of the above comparative example 1 is subjected to peeling treatment to obtain flake graphite (see Figure 3 and Figure 4 ), their thickness is still large, and the number of graphite layers is large, which does not meet the characteristics of typical graphene. However, the graphite treated by the method provided by this application (i.e., the first precipitate, see Figure 5 ), the morphology of the lamellae structure is slightly "opened", and the gaps and contours between the lamellae are more distinct. The graphene obtained after the peeling described in step (3) and the washing and drying treatment described in step (4) (see Figure 6 ), its surface has abundant wrinkles, which is a typical feature of two-dimensional materials due to their large surface energy. And the graphene appears transparent "gauze" under a transmission electron microscope (see Figure 7 ), the morphology is similar to the microscopic morphology of graphene obtained by Hummers method (ie, comparative example 2) (see Figure 8 and Figure 9 ) are very similar. This phenomenon demonstrates that graphite slag can be weakly oxidized in a mixture of acid and hydrogen peroxide at a certain concentration, causing the lamellar structure of the graphite slag to be slightly "opened," thereby facilitating graphite exfoliation and enabling the production of high-quality graphene. Furthermore, compared to the Hummers method (Comparative Example 2), the method provided in this application has simpler steps and greater controllability.

[0133] Further analysis of the microstructure of each sample: Figure 10The XRD curves of the graphite slag (a), the first precipitate (b) and the obtained graphene (c) of Example 1 of the present application are shown respectively. The graphite slag has a sharp peak near 2θ=26.5°, which is the X-ray diffraction peak of the intrinsic structure of graphite, that is, the characteristic peak of the (002) crystal plane. The (002) peak of the first precipitate is broadened and the intensity becomes smaller, and the peak position shifts to a small angle, indicating that the graphite slag undergoes oxidative intercalation in a mixed solution of acid and hydrogen peroxide, and the interlayer compounds (including oxygen-containing functional groups and intercalated substances) cause the interlayer spacing of the graphite to increase. The (002) peak of the graphene obtained in Example 1 is slightly shifted to a large angle compared to the first precipitate, indicating that after the first precipitate is stripped and washed, the oxygen-containing functional groups and intercalated substances therein fall off under the synergistic action of the stripping agent and the force field and in the subsequent washing process, which greatly reduces the structural defects in the obtained graphene.

[0134] In order to characterize the structural characteristics of the finished graphene products, Raman spectroscopy and conductivity tests were performed on graphite slag and some graphene products. Figure 11 The Raman spectra of graphite slag and graphene obtained in Example 1 and Comparative Examples 1-2 are shown. It can be seen that the D peak intensity of graphite slag is very low, indicating that the negative electrode graphite still retains the intrinsic structure of graphite after charge-discharge cycles or other aging mechanisms, and its recycling and regeneration are feasible and have high added value. The I peak of graphene obtained in Example 1 is D / I G The value is slightly higher than that of Comparative Example 1, which is caused by the introduction of oxygen-containing functional groups and intercalated molecules, resulting in a decrease in the structural integrity of graphene. In addition, the defectivity of the graphene obtained in Example 1 is still much lower than that of the graphene obtained in Comparative Example 2, which shows that the method provided by the present application is less structurally damaged than the traditional strong oxidation method, and the quality of the graphene obtained is higher. As can be seen from the test results of conductivity in Table 2, the conductivity of the graphene obtained in the embodiments is higher than that in the comparative example: compared with Comparative Example 1, the number of graphene layers in the embodiments of the present application is narrower, closer to the ideal two-dimensional material structure, and therefore has a higher conductivity; and compared with Comparative Example 2, the defect density of the graphene in the embodiments of the present application is lower, and therefore has a higher conductivity. In summary, the graphene obtained in the embodiments of the present application has the best comprehensive performance.

[0135] The batteries prepared in the Examples and Comparative Examples were subjected to electrochemical performance testing. The results in Table 3 show that the batteries assembled by adding the graphene prepared in the Examples of the present application as a conductive agent to the positive electrode active material to prepare the positive electrode sheet exhibited higher capacity and better rate performance than the batteries prepared using commercial conductive carbon black and the graphene prepared in the Comparative Examples as the positive electrode conductive agent. This indicates that the graphene prepared in the Examples of the present application can form a good conductive network in the battery's electrode sheet, thereby improving the battery's initial coulombic efficiency and the battery's charge and discharge specific capacity.

[0136] In summary, the present application provides a solution for efficiently purifying and value-added graphene from graphite slag in waste batteries. This method has simple steps, avoids the disadvantages of the redox method and other physical exfoliation methods commonly used in this field, and can produce high-quality graphene materials with a high product yield.

[0137] The above is an exemplary embodiment of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made thereto without departing from the principles of the present application. These improvements and modifications are also considered to be within the scope of protection of the present application.

Claims

1. A method for recycling graphene from waste batteries, characterized in that: The following steps are involved: (1) discharging, crushing, magnetic separation and cyclone separation of waste batteries to separate a mixture containing graphite, then acid leaching the mixture to obtain a solid, drying and calcining the solid to obtain graphite slag containing metal impurities; (2) adding the graphite slag to a mixture of an acid solution and a hydrogen peroxide solution to leach out metal impurities in the graphite slag, slightly oxidize the graphite, and introduce intercalants between the graphite layers to obtain a first dispersion; wherein the intercalants include at least one of acid molecules and water molecules; (3) subjecting the first dispersion to a solid-liquid separation treatment, collecting the first precipitate and adding it to a stripping agent, adjusting the pH value of the system, and performing a stripping treatment, so that the graphite is stripped into graphene under the combined action of the stripping agent and the intercalant, thereby obtaining a second dispersion containing graphene; wherein the stripping agent comprises any one of water, a surfactant solution, and an organic solvent; the surfactant comprises at least one of polyvinyl pyrrolidone, polyethylene glycol, polyvinyl alcohol, Tween 80, sodium cholate, sodium dodecylsulfonate, sodium linear alkylbenzenesulfonate, and sodium α-olefinsulfonate; and the organic solvent comprises at least one of N-methylpyrrolidone, ethanol, dimethylformamide, and dimethyl sulfoxide; (4) subjecting the second dispersion containing graphene to solid-liquid separation, collecting the upper layer of liquid for secondary solid-liquid separation, and collecting a second precipitate; washing and drying the second precipitate to obtain graphene powder.

2. The method according to claim 1, characterized in that The solid-liquid mass volume ratio of the graphite slag to the acid solution is 1:(5-200) g / mL.

3. The method according to claim 1, characterized in that The solid-liquid mass volume ratio of the graphite slag to the hydrogen peroxide solution is 1:(0.1-20) g / mL.

4. The method according to claim 1, wherein In step (2), the mass ratio of the solute acid in the acid solution to the hydrogen peroxide solute in the hydrogen peroxide solution is 1:(0.001-0.5).

5. The method according to claim 1, wherein In step (3), the pH value of the system is adjusted to 7-11.

6. The method according to claim 1, characterized in that The mass volume ratio of the first precipitate to the stripping agent is 1 mg / mL-300 mg / mL.

7. A graphene, characterized in that: The graphene is prepared by the method according to any one of claims 1 to 6.

8. The graphene according to claim 7, characterized in that In the Raman spectrum of the graphene, the defect characteristic peak is closely related to the sp 2 The ratio of the intensity values ​​of the in-plane vibration peaks of the carbon atoms is 0.2-0.4, and the electrical conductivity of the graphene is not less than 65000 S / m.

9. The graphene according to claim 8, characterized in that The number of layers of the graphene is 1-10.

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