Calcination carbon removal-supercritical expansion stripping method for regenerating graphene from graphite of waste lithium battery

Abstract

The invention discloses a calcining carbon removal-supercritical expansion stripping method for regenerating graphene from graphite of a waste lithium battery, and belongs to the field of lithium battery recovery and nano material preparation. Comprising the following steps: treating the waste lithium battery to obtain waste lithium battery positive and negative electrode mixed powder, and then roasting to obtain battery powder; leaching the battery powder in a nitric acid solution to obtain purified graphite slag; the method comprises the following steps: mixing graphite slag with supercritical gas fluid, treating, quickly releasing pressure to normal pressure, and separating to obtain graphene. According to the method, on the basis of interlayer structure characteristics of the waste graphite negative electrode, organic matters and amorphous carbon are removed through multi-stage calcination, a supercritical N2 / CO2 mixed fluid permeation and pressure relief expansion synergistic stripping technology is combined, and low-damage dissociation is achieved through graphite microcracks. According to the method, physical stripping and liquid nitrogen shock cooling strengthening are coupled, and the obtained graphene sheet layer is high in integrity and few in structural defect and can be directly used for manufacturing conductive composite materials and energy storage devices.

Description

Technical Field

[0001] The present invention relates to the technical field of recycling of waste batteries, and particularly to a calcination carbon removal-supercritical expansion stripping method for regenerating graphene from graphite of waste lithium batteries. Background Art

[0002] With the explosive growth of the new energy vehicle industry, the annual scrapping volume of global waste lithium batteries has exceeded 2 million tons, of which the negative electrode graphite accounts for about 15%-20%, but its recycling rate is less than 10%. Traditional chemical oxidation methods (such as the Hummers method) rely on strong oxidants such as concentrated sulfuric acid and potassium permanganate, generating a large amount of sulfur / manganese-containing wastewater during the production process, and the severe oxidation reaction causes serious damage to the graphene structure (I D / I G >1.2), making it difficult to meet the requirements of high-end applications; physical stripping methods (such as ball milling, ultrasonic) avoid chemical pollution, but have bottlenecks of low stripping efficiency (<50%) and high energy consumption (>2000 kWh / ton), and mechanical action is prone to cause interlayer slip or lattice distortion of graphite; the high-temperature graphitization method requires long-time treatment in an inert atmosphere above 1000°C, and the energy consumption accounts for 60%-70% of the total cost, and high temperature is easy to cause curling of the graphite edge, reducing the stripping efficiency. In addition, the residual binders (such as PVDF), electrolyte decomposition products and amorphous carbon (5%-10%) in waste lithium batteries are difficult to completely remove, resulting in high impurity content (>5%) in the recycled graphite, restricting the purity and performance of subsequent graphene preparation. Therefore, there is an urgent need to develop a green, efficient and low-energy physical regeneration method to break through the technical barriers of high-value recycling of negative electrode graphite. Summary of the Invention

[0003] The purpose of the present invention is to overcome the deficiencies of the above-mentioned existing technologies and provide a fully physical synergistic method for regenerating graphene from graphite of waste lithium batteries. This method abandons the traditional chemical oxidation process, and through the physical synergistic mechanism of calcination carbon removal-supercritical fluid expansion stripping-ultrasonic dispersion, realizes controllable stripping of graphite layers at low temperature, realizes efficient regeneration of graphite resources and low-defect preparation of graphene, avoids chemical reagent pollution, and at the same time improves the yield and quality of graphene.

[0004] To achieve the above purpose, the technical solutions adopted in this article are as follows: A calcination carbon removal-supercritical expansion stripping method for regenerating graphene from graphite of waste lithium batteries, comprising the following steps: Process the waste lithium battery to obtain a mixed powder of the positive and negative electrodes of the waste lithium battery, and then roast it at 400-600°C to obtain battery powder; Calcine the battery powder and then leach it in a nitric acid solution, filter, and dry to obtain purified graphite slag; Mix the purified graphite slag with supercritical gas fluid and then carry out exfoliation treatment, and then quickly release the pressure to atmospheric pressure to achieve the exfoliation of graphite layers. Separate the exfoliated product to obtain graphene.

[0005] Furthermore, the calcination is carried out in an argon atmosphere, the calcination time is 1 - 3 h, and the heating rate is 3 - 6 °C / min. The roasting is carried out in an oxygen atmosphere, the temperature is 350 - 450 °C, the roasting time is 0.5 - 1 h, and the heating rate is 5 - 10 °C / min.

[0006] Furthermore, the waste lithium batteries are processed. The specific process is as follows: Soak and discharge the waste lithium batteries in a sodium chloride solution with a concentration of 15 - 20 wt% for 24 h and then dry, and then discharge, crush, carry out magnetic separation, and pulverize and screen.

[0007] Furthermore, the leaching temperature is 70 - 90 °C, the leaching time is 3 - 5 h, and during leaching, the solid-liquid ratio is 20 - 40 g / L.

[0008] Furthermore, the supercritical gas fluid is N2 and / or CO2.

[0009] Furthermore, the ratio of graphite slag to supercritical gas is 1 g:2 - 5 mL, the treatment temperature is 35 - 50 °C, the treatment pressure is 8 - 12 MPa, and the treatment time is 2 - 4 h.

[0010] Furthermore, the process of mixing the purified graphite slag with supercritical gas fluid and then carrying out exfoliation treatment is implemented for 2 - 3 cycles, and each time after pressure release, it is repressurized to 8 - 12 MPa.

[0011] Furthermore, immediately after pressure release, the material is immersed in liquid nitrogen for quenching treatment, and the cooling rate ≥ 50 °C / min.

[0012] Furthermore, the process of separating the exfoliated product is as follows: Carry out ultrasonic treatment at a power of 200 - 400 W for 0.5 - 1 h in a protective gas atmosphere, collect the suspension after centrifugal separation, and dry.

[0013] Furthermore, the protective gas is Ar2 or N2, the drying temperature is 60 - 80 °C, the vacuum degree ≤ -0.08 MPa, and the drying time is 12 - 24 h. Compared with the prior art, the present invention has the following beneficial effects: Based on the intrinsic loose interlayer structure characteristics of the graphite anode of waste lithium batteries, this invention precisely removes residual organic matter and amorphous carbon through a two-stage temperature-controlled firing process. Combining the synergistic effects of the osmotic pressure difference regulation of supercritical N2 / CO2 mixed fluid and rapid phase change pressure relief expansion, it utilizes the microcracks in graphite itself to achieve layer-by-layer self-supporting low-damage dissociation. By innovatively coupling physical peeling and liquid nitrogen quenching strengthening technology, while avoiding the pollution of traditional strong acid oxidants, the peeling energy consumption is reduced by more than 65%. The thickness of the obtained graphene sheets can be controlled within the range of 2 - 6 layers, and the defect density is significantly reduced to I D / I G <0.8, and the integrity of the sheets reaches 98%. It can be directly used in the manufacture of high-end conductive composite materials and high-energy density energy storage devices. In terms of environmental protection and cost control, this invention adopts a closed-loop process system throughout. Through the collaborative design of the supercritical CO2 recycling process and the regeneration process of nitric acid leaching solution, the solvent utilization rate > 85% and the carbon emission equivalent are reduced by 72% compared with the electrochemical method; compared with the traditional high-temperature graphitization process, the calcination temperature is reduced by 40% and the comprehensive energy consumption is saved by 50%. Combining the metal resource recovery income, the overall production cost is reduced by more than 35%. This technology not only overcomes the technical bottleneck that it is difficult to have both high crystallinity and low damage in graphene preparation, but also provides an innovative "pyrolysis purification - supercritical peeling - low-temperature quenching - resource recovery" four-in-one solution for the green regeneration of the anode materials of power batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a flowchart of the regeneration of graphene from the graphite of waste lithium batteries; Figure 2 is an SEM image of the graphene regenerated from the graphite of waste lithium batteries obtained in Example 1; Figure 3 is a Raman image of the graphene regenerated from the graphite of waste lithium batteries obtained in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0015] To facilitate the understanding of this invention, the following will describe this invention more comprehensively with reference to the relevant drawings. The preferred embodiments of this invention are shown in the drawings. However, this invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the disclosure of this invention more thorough and comprehensive.

[0016] A calcination carbon removal - supercritical expansion peeling method for regenerating graphene from the graphite of waste lithium batteries according to this invention includes the following steps: (1)Discharge, crush, magnetically separate and pulverize and screen the waste lithium batteries to obtain a mixed powder of the positive and negative electrodes of the waste lithium batteries. Place the obtained mixed powder in a tubular furnace and calcine it in an argon atmosphere at a temperature of 400 - 600 °C for 1 - 3 h, with a heating rate set at 3 - 6 °C / min to remove the organic binder and electrolyte; Preferably, the method for discharging the waste lithium batteries in step (1) includes: soaking and discharging in a sodium chloride solution with a concentration of about 15 - 20 wt% for 24 h, then taking out the soaked waste lithium batteries, naturally drying them and putting them into a battery disassembly machine for subsequent crushing, magnetic separation and pulverizing and screening.

[0017] (2)Place the above-mentioned calcined mixed powder in a muffle furnace and calcine it at a certain temperature and for a certain time to remove the amorphous carbon therein; among them, the calcination temperature is 350 - 450 °C, the calcination time is 0.5 - 1 h, and the heating rate is 5 - 10 °C / min.

[0018] (3)Place the above-mentioned calcined battery powder in a nitric acid solution with a mass concentration of 30%, with a leaching temperature of 70 - 90 °C, a leaching time of 3 - 5 h, and a solid-liquid ratio of 20 - 40 g / L to remove metal oxides, and obtain purified graphite slag after filtration and drying; (4)Mix the graphite slag with a supercritical gas fluid according to a certain mass-volume ratio, process it at a specific temperature and pressure for a certain time, and then quickly release the pressure to atmospheric pressure. Utilize the instantaneous expansion of the supercritical fluid to achieve the exfoliation of the graphite layer and obtain an exfoliated product; among them, the supercritical gas fluid is N2 and / or CO2; the ratio of the graphite slag to the supercritical gas is 1 g:2 - 5 mL, the processing temperature is 35 - 50 °C, the pressure is 8 - 12 MPa, and the time is 2 - 4 h. The supercritical carbon dioxide treatment process is carried out 2 - 3 cycles, and the pressure is re-pressurized to 8 - 12 MPa after each pressure release. The material after pressure release is immediately immersed in liquid nitrogen for quenching treatment, and the cooling rate ≥50 °C / min.

[0019] (5)Ultrasonically process the exfoliated product in a protective gas atmosphere at a power of 200 - 400 W for 0.5 - 1 h, centrifuge and separate to collect the suspension, and vacuum dry to obtain graphene. Among them, the protective gas is Ar2 or N2, and the vacuum drying conditions are a temperature of 60 - 80 °C, a vacuum degree ≤ -0.08 MPa, and a drying time of 12 - 24 h.

[0020] The following are specific examples.

[0021] Example 1 A calcination carbon removal-supercritical expansion exfoliation method for regenerating graphene from graphite in waste lithium batteries, referring to Figure 1 as shown, includes the following steps: (1) The recycled 10 kg of retired lithium iron phosphate batteries were soaked in a 20 wt% sodium chloride solution. After discharging until the voltage was lower than 1.5 V, they were taken out and dried. Subsequently, the batteries were put into a machine for shredding and crushing, and the volume of the crushed product was about 5% of the original. The crushed pieces were separated by air separation and magnetic separation to remove the diaphragm, magnetic current collectors, and metallic elements; then they were pulverized and screened to separate the mixed powder from the aluminum foil and copper foil, and finally the pretreated mixed powder was obtained.

[0022] (2) The obtained mixed powder was placed in a tubular furnace and calcined at 500 °C for 3 h in an argon atmosphere, and the heating rate was set at 5 °C / min to remove the organic binder and electrolyte; (3) The above-mentioned calcined mixed powder was placed in a muffle furnace. Under an oxygen atmosphere, it was heated to 350 °C at a rate of 5 °C / min and then calcined for 0.5 h to remove the amorphous carbon in it; (4) The above-mentioned calcined battery powder was placed in a nitric acid solution with a mass concentration of 30%. The leaching temperature was 75 °C, the leaching time was 3 h, and the solid-liquid ratio was 25 g / L. After filtration and drying, purified graphite slag was obtained; (5) The graphite slag and supercritical CO2 fluid were mixed at a mass-to-volume ratio of 1 g:2 mL and treated at 40 °C and 10 MPa for 2 h, and then quickly depressurized to atmospheric pressure. The depressurized material was immediately immersed in liquid nitrogen for rapid cooling treatment, and the cooling rate was 50 °C / min. Then it was pressurized to 10 MPa again. The above process was repeated 2 times, and the exfoliation of graphite layers was achieved by the instantaneous expansion of the supercritical fluid; (6) The exfoliated product was ultrasonically treated at a power of 400 W for 0.5 h in an N2 atmosphere, and the suspension was collected after centrifugation and vacuum dried at 60 °C for 12 h to obtain graphene.

[0023] See Figure 2 , it can be seen that the graphene prepared in Example 1 presents a two-dimensional gauze-like sheet structure with wrinkled and curly characteristics, showing its light and thin characteristics and large specific surface area.

[0024] See Figure 3 , it can be seen that the graphene I D / I G value of the graphene prepared in Example 1 is only 0.151, indicating that it has fewer structural defects and higher order. At the same time, the Raman spectrum shows the presence of a 2D peak, and I 2D / I G is 0.5025, proving that this graphene is few-layer graphene.

[0025] Example 2 A method for calcination carbon removal-supercritical expansion exfoliation of recycled graphene from waste lithium batteries, referring to Figure 1As shown in the figure, it includes the following steps: (1) The recycled 10 kg of retired lithium iron phosphate batteries are soaked in 18 wt% sodium chloride solution. After discharging until the voltage is lower than 1.5 V, they are taken out and dried. Subsequently, the batteries are put into a machine for shredding and crushing, and the volume of the crushed materials is about 5% of the original. The crushed pieces are separated by air separation and magnetic separation to remove the diaphragm, magnetic ear tabs and metallic elements therein; then they are pulverized and screened to separate the mixed powder from aluminum foil and copper foil, and finally the pretreated mixed powder is obtained.

[0026] (2) The obtained mixed powder is placed in a tubular furnace and calcined at 450 °C for 2 h in an argon atmosphere, and the heating rate is set at 4 °C / min to remove the organic binder and electrolyte; (3) The above-calcined mixed powder is placed in a muffle furnace and heated to 350 °C at a rate of 5 °C / min in an oxygen atmosphere and then calcined for 0.5 h to remove the amorphous carbon therein; (4) The above-calcined battery powder is placed in a nitric acid solution with a mass concentration of 30%, the leaching temperature is 85 °C, the leaching time is 4 h, and the solid-liquid ratio is 20 g / L. After filtration and drying, purified graphite slag is obtained; (5) The graphite slag and supercritical CO2 fluid are mixed at a mass-to-volume ratio of 1 g:5 mL, treated at 35 °C and 8 MPa for 2 h, and then quickly depressurized to atmospheric pressure. The depressurized material is immediately immersed in liquid nitrogen for quenching treatment, and the cooling rate is 55 °C / min. Then it is repressurized to 8 MPa, and the above process is repeated 2 times. The instantaneous expansion of the supercritical fluid is used to achieve the exfoliation of graphite layers; (6) The exfoliated product is ultrasonically treated at a power of 200 W in an N2 atmosphere for 1 h, the suspension is collected after centrifugal separation, and vacuum dried at 60 °C for 12 h to obtain graphene.

[0027] Example 3 A method for calcining to remove carbon and supercritical expansion exfoliation of recycled graphene from waste lithium batteries, referring to Figure 1 As shown in the figure, it includes the following steps: (1) The recycled 10 kg of retired lithium iron phosphate batteries are soaked in 20 wt% sodium chloride solution. After discharging until the voltage is lower than 1.5 V, they are taken out and dried. Subsequently, the batteries are put into a machine for shredding and crushing, and the volume of the crushed materials is about 5% of the original. The crushed pieces are separated by air separation and magnetic separation to remove the diaphragm, magnetic ear tabs and metallic elements therein; then they are pulverized and screened to separate the mixed powder from aluminum foil and copper foil, and finally the pretreated mixed powder is obtained.

[0028] (2) Place the obtained mixed powder in a tubular furnace and calcine it at 450 °C for 2 h in an argon atmosphere, with the heating rate set at 4 °C / min, to remove the organic binder and electrolyte; (3) Place the above calcined mixed powder in a muffle furnace, and under an oxygen atmosphere, heat it to 350 °C at a rate of 8 °C / min and then calcine it for 0.5 h to remove the amorphous carbon therein; (4) Place the above calcined battery powder in a nitric acid solution with a mass concentration of 30%, with the leaching temperature at 90 °C, the leaching time at 3 h, and the solid-liquid ratio at 20 g / L. After filtration and drying, obtain purified graphite slag; (5) Mix the graphite slag and supercritical CO2 fluid at a mass-volume ratio of 1 g:5 mL, treat it at 45 °C and 8 MPa for 2 h, and then quickly release the pressure to atmospheric pressure. Immediately immerse the depressurized material in liquid nitrogen for quenching treatment, with the cooling rate at 50 °C / min, and then repressurize it to 8 MPa. The above process is repeated 2 times to achieve the exfoliation of graphite layers by the instantaneous expansion of the supercritical fluid; (6) Ultrasonically treat the exfoliated product at a power of 200 W for 1 h in an N2 atmosphere, collect the suspension after centrifugal separation, and vacuum dry it at 60 °C for 12 h to obtain graphene.

[0029] Example 4 A method for calcination carbon removal-supercritical expansion exfoliation of recycled graphene from graphite in waste lithium batteries, as shown in Figure 1 the following steps: (1) Immerse 10 kg of recycled retired lithium iron phosphate batteries in a 20 wt% sodium chloride solution, discharge until the voltage is lower than 1.5 V, then take them out and dry. Subsequently, put the batteries into a machine for shredding and crushing, and the volume of the crushed product is about 5% of the original. The crushed pieces are separated by air separation and magnetic separation to remove the diaphragm and magnetic current collectors and metallic elements therein; then perform pulverization and screening to separate the mixed powder from the aluminum foil and copper foil, and finally obtain the pretreated mixed powder.

[0030] (2) Place the obtained mixed powder in a tubular furnace and calcine it at 600 °C for 2 h in an argon atmosphere, with the heating rate set at 5 °C / min, to remove the organic binder and electrolyte; (3) Place the above calcined mixed powder in a muffle furnace, and under an oxygen atmosphere, heat it to 400 °C at a rate of 5 °C / min and then calcine it for 0.5 h to remove the amorphous carbon therein; (4) Place the above calcined battery powder in a nitric acid solution with a mass concentration of 30%, with the leaching temperature at 75 °C, the leaching time at 4 h, and the solid-liquid ratio at 20 g / L. After filtration and drying, obtain purified graphite slag; (5) Mix the graphite slag with supercritical N2 fluid at a mass-to-volume ratio of 1 g:3 mL, treat it at 50 °C and 10 MPa for 2 h, and then quickly release the pressure to atmospheric pressure. Immediately immerse the depressurized material in liquid nitrogen for quenching treatment, with a cooling rate of 50 °C / min, and then repressurize it to 10 MPa. The above process is repeated 2 times to achieve graphite layer exfoliation using the instantaneous expansion of the supercritical fluid; (6) Ultrasonically treat the exfoliated product in an N2 atmosphere at a power of 200 W for 1 h, collect the suspension after centrifugal separation, and vacuum dry it at 70 °C for 18 h to obtain graphene.

[0031] Example 5 A method for calcining carbon removal and supercritical expansion exfoliation of recycled graphene from graphite in waste lithium batteries, referring to Figure 1 as shown, includes the following steps: (1) Immerse 10 kg of recycled lithium iron phosphate waste batteries in a 20 wt% sodium chloride solution, discharge until the voltage is lower than 1.5 V, then take them out and dry. Subsequently, put the batteries into a machine for shredding and crushing, and the volume of the crushed product is about 5% of the original. The crushed pieces are separated by air separation and magnetic separation to remove the diaphragm, magnetic current collectors, and metallic elements; then they are pulverized and screened to separate the mixed powder from the aluminum foil and copper foil, and finally the pretreated mixed powder is obtained.

[0032] (2) Place the obtained mixed powder in a tube furnace, roast it in an argon atmosphere at a temperature of 600 °C for 1 h, and set the heating rate to 6 °C / min to remove the organic binder and electrolyte; (3) Place the roasted mixed powder in a muffle furnace, heat it to 350 °C at a rate of 5 °C / min in an oxygen atmosphere, and then calcine it for 1 h to remove the amorphous carbon in it; (4) Place the calcined battery powder in a nitric acid solution with a mass concentration of 30%, with a leaching temperature of 75 °C, a leaching time of 4 h, and a solid-to-liquid ratio of 20 g / L. After filtration and drying, purified graphite slag is obtained; (5) Mix the graphite slag with supercritical N2 fluid at a mass-to-volume ratio of 1 g:3 mL, treat it at 40 °C and 10 MPa for 3 h, and then quickly release the pressure to atmospheric pressure. Immediately immerse the depressurized material in liquid nitrogen for quenching treatment, with a cooling rate of 50 °C / min, and then repressurize it to 10 MPa. The above process is repeated 2 times to achieve graphite layer exfoliation using the instantaneous expansion of the supercritical fluid; (6) Ultrasonically treat the exfoliated product in an N2 atmosphere at a power of 200 W for 1 h, collect the suspension after centrifugal separation, and vacuum dry it at 80 °C for 18 h to obtain graphene.

[0033] Example 6 A calcination carbon removal-supercritical expansion stripping method for regenerating graphene from graphite in waste lithium batteries, refer to Figure 1 as shown below, including the following steps: (1) The recycled 10 kg of retired lithium iron phosphate batteries are immersed in a 20 wt% sodium chloride solution. After discharging to a voltage below 1.5 V, they are taken out and dried. Subsequently, the batteries are put into a machine for shredding and crushing, and the volume of the crushed material is about 5% of the original. The crushed pieces are separated by air separation and magnetic separation to remove the diaphragm, magnetic current collectors and metallic elements therein; then, they are pulverized and screened to separate the mixed powder from the aluminum foil and copper foil, and finally the pretreated mixed powder is obtained.

[0034] (2) The obtained mixed powder is placed in a tubular furnace and calcined in an argon atmosphere at a temperature of 550 °C for 1 h, and the heating rate is set at 5 °C / min to remove the organic binder and electrolyte; (3) The mixed powder after the above calcination is placed in a muffle furnace. Under an oxygen atmosphere, it is heated to 400 °C at a rate of 10 °C / min and then calcined for 0.5 h to remove the amorphous carbon therein; (4) The battery powder after the above calcination is placed in a nitric acid solution with a mass concentration of 30%. The leaching temperature is 80 °C, the leaching time is 4 h, and the solid-liquid ratio is 40 g / L. After filtration and drying, purified graphite slag is obtained; (5) The graphite slag is mixed with supercritical N2 fluid at a mass-to-volume ratio of 1 g:4 mL, and treated at 45 °C and 8 MPa for 2 h, and then quickly depressurized to atmospheric pressure. The material after depressurization is immediately immersed in liquid nitrogen for quenching treatment, and the cooling rate is 50 °C / min. Then, it is pressurized to 8 MPa again, and the above process is repeated 3 times. The instantaneous expansion of the supercritical fluid is used to achieve the exfoliation of graphite layers; (6) The exfoliated product is ultrasonically treated at a power of 200 W in an Ar2 atmosphere for 1 h, and the suspension is collected after centrifugal separation and vacuum dried at 80 °C for 12 h to obtain graphene.

[0035] Example 7 A calcination carbon removal-supercritical expansion stripping method for regenerating graphene from graphite in waste lithium batteries, refer to Figure 1 as shown below, including the following steps: (1) The recycled 10 kg of retired lithium iron phosphate batteries are immersed in a 15 wt% sodium chloride solution. After discharging to a voltage below 1.5 V, they are taken out and dried. Subsequently, the batteries are put into a machine for shredding and crushing, and the volume of the crushed material is about 5% of the original. The crushed pieces are separated by air separation and magnetic separation to remove the diaphragm, magnetic current collectors and metallic elements therein; then, they are pulverized and screened to separate the mixed powder from the aluminum foil and copper foil, and finally the pretreated mixed powder is obtained.

[0036] (2) Place the obtained mixed powder in a tubular furnace and calcine it at 400 °C for 1 h in an argon atmosphere. Set the heating rate to 3 °C / min to remove the organic binder and electrolyte; (3) Place the above calcined mixed powder in a muffle furnace. Under an oxygen atmosphere, heat it to 450 °C at a rate of 6 °C / min and then calcine it for 0.7 h to remove the amorphous carbon therein; (4) Place the above calcined battery powder in a nitric acid solution with a mass concentration of 30%. The leaching temperature is 70 °C, the leaching time is 5 h, and the solid-liquid ratio is 30 g / L. After filtration and drying, purified graphite slag is obtained; (5) Mix the graphite slag with supercritical N2 fluid at a mass-to-volume ratio of 1 g:4 mL. Treat it at 30 °C and 12 MPa for 4 h, and then quickly release the pressure to atmospheric pressure. Immediately immerse the depressurized material in liquid nitrogen for quenching treatment. The cooling rate is 50 °C / min, and then repressurize it to 12 MPa. The above process is repeated 3 times to achieve graphite layer exfoliation by the instantaneous expansion of the supercritical fluid; (6) Ultrasonically treat the exfoliated product at a power of 300 W for 0.8 h in an Ar2 atmosphere. After centrifugal separation, collect the suspension and vacuum dry it at 65 °C for 24 h to obtain graphene.

[0037] The parameters of the graphene prepared in Examples 1-6 are shown in Table 1.

[0038] Table 1 Comparison table of relevant parameters of graphene prepared in each example

[0039] As can be seen from Table 1, the yield of graphene prepared by the method of the present invention is higher than 80%. At the same time, it has fewer layers (<10 layers), a lower I D / I G value (<0.2), a smaller powder tablet resistance (<20 mΩ·cm), and a higher specific surface area (>150 m 2 / g), highlighting its possible high productivity, material stability, conductivity, and high capacity. These characteristics are all key performance indicators expected in application fields such as conductive composite materials and energy storage devices.

[0040] Compared with the existing methods for preparing graphene, the advantages of the present invention are shown in Table 2.

[0041] Table 2 Comparison between the supercritical CO2 exfoliation method for preparing graphene of the present invention and the existing methods for preparing graphene

[0042] Supercritical CO2 (31 °C, 7.38 MPa) has properties similar to non-polar solvents such as ethane and can dissolve non-polar or slightly polar compounds. The presence of a small amount of co-solvent (such as alcohols) will further increase its solubility limit.

[0043] Table 3 Key data of selected solvents for graphene processing

[0044] As can be seen from Table 3, among various supercritical fluid systems, the CO2 medium exhibits significant thermodynamic advantages. Its critical temperature is not only lower than that of traditional organic solvents, but also significantly reduces the temperature of the graphene exfoliation reaction system. This not only greatly reduces the energy consumption in graphene preparation, but also avoids damage to the graphene lattice structure during the high-temperature process, providing a better material basis for subsequent device processing.

[0045] In the present invention, taking advantage of the relatively loose structure of the graphite in the negative electrode of waste lithium batteries, physical exfoliation of graphite layers is achieved under mild conditions using supercritical N2 / CO2 fluid, combined with rapid quenching in liquid nitrogen to fix the exfoliated structure, avoiding the use of chemical intercalating agents, and reducing the graphene defect density to I D / I G <0.8; there is no strong oxidant throughout the present invention, and nitric acid can be recycled after acid leaching, reducing the pollution risk; compared with the traditional high-temperature graphitization process, the calcination temperature of the present invention is reduced by 40% and the energy consumption is saved by 50%. At the same time, supercritical CO2 can be recycled, and metal can also be recovered from the nitric acid leaching solution, reducing the comprehensive cost by 35%.

[0046] The above is only an illustration of the best embodiments of the present invention, but it should not be construed as a limitation of the claims. The present invention is not limited to the above embodiments, and its specific structure is allowed to vary. Any changes made within the protection scope of the independent claims of the present invention are within the protection scope of the present invention.

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the description of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

Claims

1. A calcination carbon removal-supercritical expansion stripping method for regenerating graphene from graphite in waste lithium batteries, characterized in that, It includes the following steps: Treat the waste lithium batteries to obtain the mixed powder of the positive and negative electrodes of the waste lithium batteries, and then calcine at 400 - 600 °C to obtain battery powder materials; Calcine the battery powder materials and then leach them in a nitric acid solution, filter, and dry to obtain purified graphite slag; Mix the purified graphite slag with supercritical gas fluid and then carry out a peeling treatment, and then quickly depressurize to atmospheric pressure to achieve graphite layer peeling; Separate the peeling products to obtain graphene.

2. The calcination carbon removal-supercritical expansion stripping method for recycling graphene from waste lithium battery graphite according to claim 1, characterized in that, The calcination is carried out in an argon atmosphere, the calcination time is 1 - 3 h, and the heating rate is 3 - 6 °C / min; The calcination is carried out in an oxygen atmosphere, the temperature is 350 - 450 °C, the calcination time is 0.5 - 1 h, and the heating rate is 5 - 10 °C / min.

3. The calcination carbon removal-supercritical expansion stripping method for recycling graphene from waste lithium battery graphite according to claim 1, wherein, Treat the waste lithium batteries, and the specific process is: soak and discharge the waste lithium batteries in a sodium chloride solution with a concentration of 15 - 20 wt% for 24 h and then dry, and then discharge, crush, magnetically separate, and pulverize and screen.

4. The calcination carbon removal-supercritical expansion stripping method for recycling graphene from graphite in waste lithium batteries according to claim 1, characterized in that, The leaching temperature is 70 - 90 °C, the leaching time is 3 - 5 h, and during leaching, the solid-liquid ratio is 20 - 40 g / L.

5. The calcination carbon removal-supercritical expansion stripping method for recycling graphene from waste lithium battery graphite according to claim 1, characterized in that, The supercritical gas fluid is N2 and / or CO2.

6. The calcination carbon removal-supercritical expansion peeling method for recycling graphene from graphite in waste lithium batteries according to claim 1, characterized in that, The ratio of graphite slag to supercritical gas is 1 g:2 - 5 mL, the treatment temperature is 35 - 50 °C, the treatment pressure is 8 - 12 MPa, and the treatment time is 2 - 4 h.

7. The calcination carbon removal-supercritical expansion stripping method for recycling graphene from graphite in waste lithium batteries according to claim 1, characterized in that Carry out the peeling treatment process by mixing the purified graphite slag with supercritical gas fluid for 2 - 3 cycles, and re-pressurize to 8 - 12 MPa after each depressurization.

8. The calcination carbon removal-supercritical expansion stripping method for recycling graphene from graphite in waste lithium batteries according to claim 1, wherein, Immediately immerse the material after depressurization in liquid nitrogen for quenching treatment, and the cooling rate ≥ 50 °C / min.

9. The calcination carbon removal-supercritical expansion peeling method for regenerating graphene from graphite in waste lithium batteries according to claim 1, wherein, Separate the peeling products, and the specific process is: carry out ultrasonic treatment at a power of 200 - 400 W for 0.5 - 1 h in a protective gas atmosphere, centrifuge and separate, and collect the suspension and dry.

10. The calcination carbon removal-supercritical expansion peeling method for recycling graphene from graphite in waste lithium batteries according to claim 9, wherein, The protective gas is Ar2 or N2, the drying temperature is 60 - 80 °C, the vacuum degree ≤ -0.08 MPa, and the drying time is 12 - 24 h.

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