A method for purifying graphite by cascade plasma and high-purity low-damage graphite powder

Abstract

The application provides a method for purifying graphite by using a gradient plasma and high-purity low-damage graphite powder. The method comprises the following steps: performing first gradient plasma treatment on graphite raw materials to obtain a first intermediate product; performing second gradient plasma treatment on the first intermediate product in a first mixed gas with an oxygen-containing gas content of less than or equal to 5 vol% to obtain a second intermediate product; performing third gradient plasma treatment on the second intermediate product in a second mixed gas with a fluorocarbon gas content of less than or equal to 1.5 vol% to obtain a third intermediate product; performing fourth gradient plasma treatment on the third intermediate product in a third mixed gas with a reducing gas content of greater than or equal to 1 vol% to obtain a fourth intermediate product; and performing washing on the fourth intermediate product to obtain high-purity low-damage graphite powder. The high-purity low-damage graphite powder obtained by the method has a high fixed carbon content and fewer defects in the graphite structure.

Description

Technical Field

[0001] This invention belongs to the field of high-purity graphite preparation technology, specifically relating to a method for purifying graphite using a step-by-step plasma process and high-purity, low-damage graphite powder. Background Technology

[0002] Graphite, a material with excellent thermal conductivity, electrical conductivity, lubrication, and high-temperature resistance, is widely used in high-end fields such as semiconductor chips, electronic components, and high-temperature equipment. These fields require extremely high purity graphite, typically exceeding 99.99%. Currently, graphite purification methods mainly include physical purification, chemical purification, high-temperature purification, and plasma purification.

[0003] Among them, plasma purification has become a research hotspot in the preparation of high-purity graphite due to its advantages such as relatively low processing temperature, high impurity removal efficiency, and minimal damage to the graphite structure. Existing plasma graphite purification technologies mostly employ a single atmosphere or single-stage treatment, such as directly using Freon plasma for impurity removal, or using oxygen plasma for layer opening followed by direct fluoride impurity removal. However, these plasma graphite purification technologies have several problems: First, if the oxygen ratio is not properly controlled during oxygen plasma layer opening, it can easily lead to oxidation and ablation of the graphite substrate, causing carbon loss and structural collapse; second, excessive Freon usage can produce a large amount of highly toxic byproducts, not only endangering operator safety but also increasing exhaust gas treatment costs and polluting the environment; third, single-stage impurity removal cannot achieve comprehensive impurity removal from the surface, interlayer, and lattice, easily leading to impurity residues and affecting graphite purity; fourth, graphite after impurity removal has many structural defects, affecting its key properties such as thermal conductivity and electrical conductivity.

[0004] Therefore, developing a plasma graphite purification method that can precisely control each stage of the reaction, thoroughly remove impurities, minimize damage to the graphite structure, and ensure high safety has become a pressing technical challenge. Summary of the Invention

[0005] To address the aforementioned technical problems, the present invention aims to provide a method for purifying graphite using a step-by-step plasma process and a high-purity, low-damage graphite powder. This high-purity, low-damage graphite powder has a high fixed carbon content and fewer defects in its graphite structure.

[0006] To achieve the above objectives, the present invention provides a method for purifying graphite using a step-by-step plasma process, wherein the method includes: S1. The graphite raw material is subjected to a first-stage plasma treatment to obtain a first intermediate product; the first-stage plasma treatment is carried out in a protective gas. S2. In the first mixed gas, the first intermediate product is subjected to a second-stage plasma treatment to obtain a second intermediate product; the first mixed gas contains 0.1 vol%-5 vol% of oxidizing gas, and the remainder is protective gas; the oxidizing gas includes one or more of O2, O3, and N2O. S3. In the second mixed gas, the second intermediate product is subjected to a third-stage plasma treatment to obtain a third intermediate product; the second mixed gas contains 0.1 vol%-1.5 vol% of fluorocarbon gas, with the remainder being protective gases; the fluorocarbon gas includes 1,1,1,2-tetrafluoroethane and / or trifluoromethane. S4. In the third mixed gas, the third intermediate product is subjected to a fourth-stage plasma treatment to obtain a fourth intermediate product; the third mixed gas contains 1 vol%-10 vol% of reducing gas, and the remainder is protective gas; the reducing gas includes one or more of H2, NH3, and CH4. S5. Wash the fourth intermediate product to obtain high-purity, low-damage graphite powder. The radio frequency power of the first-stage plasma treatment is 50-120 W; the radio frequency power of the second-stage plasma treatment is 150-230 W; the radio frequency power of the third-stage plasma treatment is 250-350 W; and the radio frequency power of the fourth-stage plasma treatment is 120-150 W.

[0007] According to a specific embodiment of the present invention, preferably, the temperature of the first-stage plasma treatment is 100-500℃ (more preferably 120-300℃, and even more preferably 150-250℃), and the time of the first-stage plasma treatment is 5-60 min (more preferably 10-30 min, and even more preferably 10-20 min).

[0008] In some specific implementations, preferably, the radio frequency power of the first-stage plasma treatment is 65-110W, more preferably 80-100W, and even more preferably 90-95W. During the first-stage plasma treatment, low-power radio frequency plasma is used to physically sputter and desorb the graphite surface, which can remove adsorbed moisture, organic matter, and light oxides from the surface.

[0009] According to a specific embodiment of the present invention, preferably, the temperature of the second plasma treatment is 200-800℃ (more preferably 250-550℃, and even more preferably 300-400℃), and the time of the second plasma treatment is 2-30 min (more preferably 5-20 min, and even more preferably 5-10 min).

[0010] In some specific embodiments, preferably, the radio frequency power of the second-stage plasma treatment is 150-200W, more preferably 150-180W, and even more preferably 160-170W. During the second-stage plasma treatment, the use of medium-power radio frequency plasma to selectively etch graphite edge defects can generate oxygen-containing functional groups to expand the interlayer spacing.

[0011] According to a specific embodiment of the present invention, preferably, the content of oxidizing gas in the first mixed gas is 0.5 vol%-3 vol%, more preferably 0.5 vol%-1.5 vol%.

[0012] According to a specific embodiment of the present invention, preferably, the temperature of the third plasma treatment is 500-1000℃, more preferably 550-800℃, and even more preferably 550-650℃.

[0013] In some specific embodiments, preferably, the radio frequency power of the third-stage plasma treatment is 250-320W, more preferably 250-300W, and even more preferably 280-300W. During the third-stage plasma treatment, high-power radio frequency plasma is used to process the graphite in stages. By causing the generated fluorine free radicals to react with the metallic / non-metallic impurities within the graphite lattice, volatile fluorides can be generated and discharged.

[0014] According to a specific embodiment of the present invention, preferably, the third-stage plasma treatment includes at least two stages, each stage having a treatment time of 2-30 min (more preferably 2-25 min, further preferably 5-15 min, and even more preferably 5-10 min). After the previous stage is completed, there is a pause of 1-10 min (more preferably 2-8 min, and even more preferably 3-5 min) and a protective gas is introduced for purging before proceeding to the next stage.

[0015] According to a specific embodiment of the present invention, preferably, the content of fluorocarbon gas in the second mixed gas is 0.1 vol%-1 vol%, more preferably 0.1 vol%-0.5 vol%. In the third-stage plasma treatment process, the present invention adopts a method of using trace amounts of fluorocarbon gas in combination with staged treatment and intermediate protective gas purging, which can significantly reduce the generation of highly toxic byproducts (HF, CF4), thereby reducing harmful gas emissions and mitigating environmental pollution and safety risks to operators.

[0016] According to a specific embodiment of the present invention, preferably, the temperature of the fourth plasma treatment is 400-800℃ (more preferably 450-700℃, and even more preferably 500-600℃), and the time of the fourth plasma treatment is 5-60 min (more preferably 10-40 min, and even more preferably 10-20 min).

[0017] In some specific embodiments, preferably, the radio frequency power of the fourth plasma treatment is 130-150W, more preferably 130-145W, and even more preferably 140-145W. During the fourth plasma treatment, using medium-to-low power radio frequency plasma to anneal the graphite can repair lattice defects generated in previous steps and remove residual fluorine groups and oxygen-containing groups.

[0018] According to a specific embodiment of the present invention, preferably, the content of reducing gas in the third mixture is 1 vol%-6 vol%, more preferably 3 vol%-5 vol.

[0019] In some specific implementations, preferably, in steps S1, S2, S3, and S4, the protective gas includes one or more combinations of Ar, N2, and He.

[0020] In some specific embodiments, preferably, the first mixture includes one or more of the following: a mixture of Ar and O2, a mixture of N2 and O2, and a mixture of He and O2.

[0021] In some specific embodiments, preferably, the second mixed gas includes one or more of the following: a mixture of Ar and fluorocarbon gas, a mixture of N2 and fluorocarbon gas, and a mixture of He and fluorocarbon gas.

[0022] In some specific embodiments, preferably, the third mixture includes one or more of the following: a mixture of Ar and H2, a mixture of N2 and H2, and a mixture of He and H2.

[0023] According to a specific embodiment of the present invention, preferably, before step S1, a pretreatment step of acid washing, water washing and drying of the graphite raw material is included.

[0024] According to a specific embodiment of the present invention, preferably, the graphite raw material is pickled using an HCl solution with a concentration of 0.5wt%-15wt%; more preferably, the concentration of the HCl solution is 1wt%-15wt%, further preferably 1wt%-10wt%, and even more preferably 3wt%-5wt%.

[0025] In some specific embodiments, preferably, the pickling process includes: immersing the graphite raw material in an HCl solution at room temperature for 1-10 h (more preferably 1-5 h, and even more preferably 1-3 h).

[0026] In some specific implementations, preferably, in step S0, the washing conditions are: repeated washing with deionized water until the pH of the washing solution is 6.5-7.0, and the washing is completed.

[0027] In some specific implementations, preferably, step S5 specifically includes: ultrasonically washing, centrifuging and drying the fourth intermediate product to obtain high-purity graphite powder.

[0028] In some specific implementations, preferably, in steps S0 and S5, the drying conditions are: vacuum degree of 0.1-0.2 MPa, drying temperature of 120-180℃, and drying time of 10-20 h.

[0029] In some specific implementations, preferably, in step S5, the ultrasonic water washing process includes: placing the treated graphite in deionized water and performing ultrasonic dispersion for 5-10 minutes.

[0030] In some specific embodiments, preferably, in step S5, the centrifugal separation process includes: placing the dispersion in a centrifuge for solid-liquid separation, with a centrifugation speed of 6000-7000 rpm / min and a centrifugation time of 5-10 min.

[0031] In some specific implementations, preferably, in step S5, the ultrasonic water washing and centrifugal separation process needs to be repeated 3-5 times.

[0032] According to a specific embodiment of the present invention, preferably, the graphite raw material is natural graphite with a fixed carbon content of 95%-99%.

[0033] The method for purifying graphite using tiered plasma provided by this invention achieves the dual goals of comprehensive impurity removal from the "surface-interlayer-lattice" of graphite and low-damage protection of the graphite layered structure through the synergistic effect of four tiers of plasma treatment, combined with acid washing pretreatment and water washing posttreatment steps. Ultimately, it enables the preparation of high-purity, low-damage graphite with high purity and low total impurity under relatively mild conditions. Specifically, in the first-stage plasma treatment, relying on the physical sputtering effect of low-frequency protective gas plasma, moisture, organic residues, and light oxide dust on the graphite surface can be efficiently desorbed without damaging the graphite structure, laying a clean foundation for subsequent impurity removal. In the second-stage plasma treatment, by precisely controlling the composition and processing parameters of the first mixed gas, the graphite edge defect sites can be etched and oxygen-containing groups can be generated, gently expanding the interlayer spacing, while improving the activity of interlayer impurities and avoiding the graphite substrate oxidation and ablation caused by conventional oxygen plasma treatment. In the third-stage plasma treatment, the second mixed gas is used as the reaction atmosphere. Combined with staged treatment and intermediate protective gas purging, the active particles generated by the dissociation of fluorocarbon gas can precisely penetrate into the interlayer and lattice, reacting with difficult-to-remove impurities to generate low-boiling-point fluorides for volatilization and removal. In the fourth-stage plasma treatment, with the high-temperature annealing effect of the third mixed gas plasma, the graphite structural defects generated in the previous treatment process can be repaired and the crystallinity improved, while residual fluorine groups and oxygen-containing groups can be removed, thereby further improving the graphite purity. Therefore, by precisely adapting and coordinating four tiered processes, combined with impurity pre-removal in pretreatment and residual cleaning in posttreatment, this invention can ultimately achieve efficient, high-purity, and low-damage purification of graphite.

[0034] This invention also provides a high-purity, low-damage graphite powder, wherein the high-purity, low-damage graphite powder is prepared by the above-mentioned stepwise plasma purification method for graphite; the high-purity, low-damage graphite powder has a fixed carbon content ≥ 99.88% (preferably ≥ 99.995%, more preferably ≥ 99.997%), an ash content ≤ 0.11% (preferably < 0.005%, more preferably < 0.003%), and a defect density I... D / I G The ratio is ≤ 0.65 (preferably < 0.13, more preferably ≤ 0.11).

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The method for purifying graphite by plasma in stages provided by the present invention adopts a four-stage progressive plasma treatment, relying on a process that is gentle and controllable from the surface to the inside. This avoids the problems of graphite surface oxidation and ablation and structural collapse caused by parameter imbalance in single plasma treatment. It also achieves all-round impurity removal of graphite from the surface to the interlayer to the lattice, which can effectively improve the purity of graphite and solve the problems of easy oxidation of graphite, incomplete removal of impurities, and too many defects in the existing plasma purification process.

[0036] (2) The high-purity, low-damage graphite powder provided by this invention has a layered structure with minimal damage and low defect density. The process parameters are controllable and highly flexible, making it suitable for the preparation of high-purity graphite required in fields such as semiconductors and electronic devices. In addition, the amount of highly toxic byproducts generated during the purification process can be significantly reduced, thereby reducing harmful gas emissions and mitigating environmental pollution and safety risks to operators.

[0037] (3) In the step-by-step plasma purification method for graphite provided by the present invention, the process parameters of each step can be flexibly adjusted according to the initial purity of graphite to adjust the processing time, thereby adapting to graphite purification with different purity requirements. It does not require complex high-temperature and high-pressure equipment. Compared with the traditional high-temperature halogen purification method, it has lower energy consumption and simpler equipment requirements. Attached Figure Description

[0038] Figure 1 The X-ray diffraction patterns are of the high-purity, low-damage graphite powder obtained in Example 1, the graphite powder obtained in Comparative Example 1, and the graphite raw material used in Example 1.

[0039] Figure 2 The X-ray diffraction patterns are those of the high-purity, low-damage graphite powders obtained from Examples 2, 3, and 4.

[0040] Figure 3 The images show the Raman spectra of the high-purity, low-damage graphite powder obtained in Example 1, the graphite powder obtained in Comparative Example 1, and the graphite raw material used in Example 1.

[0041] Figure 4 Raman spectra of the high-purity, low-damage graphite powders obtained from Examples 2, 3, and 4. Detailed Implementation

[0042] In order to provide a clearer understanding of the technical features, objectives and beneficial effects of the present invention, the technical solution of the present invention will now be described in detail below, but it should not be construed as limiting the scope of implementation of the present invention.

[0043] It should be noted that, unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0044] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0045] It should be understood that the terms “comprising,” “including,” and / or “containing” as used herein specify the presence of the stated features, integers, steps, components, or combinations thereof, but do not exclude the presence or addition of one or more other features, integers, steps, components, or combinations thereof.

[0046] 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.

[0047] Example 1: This embodiment provides a method for purifying graphite using a step-by-step plasma method, the specific steps of which are as follows: S0. Natural graphite raw material with a fixed carbon content of 96% was immersed in a 5wt% HCl solution at room temperature for 2 hours for acid washing, and then repeatedly washed with deionized water until the pH of the washing solution was 6.8. Finally, it was vacuum dried at 0.1MPa and 150℃ for 12 hours to obtain pretreated graphite raw material. S1. Under an Ar atmosphere, the pretreated graphite raw material is subjected to a first-stage plasma treatment to cause physical sputtering desorption on the graphite surface, thereby obtaining the first intermediate product; wherein the radio frequency power is a low-power radio frequency of 90W, the treatment temperature is 200℃, and the treatment time is 15min. S2. Under a first mixed gas atmosphere with an O2 content of 1 vol% and the remainder being Ar, the first intermediate product is subjected to a second-stage plasma treatment to selectively etch graphite edge defects, thereby obtaining the second intermediate product; wherein the radio frequency power is a medium-power radio frequency of 160W, the processing temperature is 350℃, and the processing time is 8min. S3. Under a second mixed gas atmosphere containing 0.3 vol% 1,1,1,2-tetrafluoroethane and the remainder Ar, the second intermediate product is subjected to a third-stage plasma treatment to obtain a third intermediate product. The third-stage plasma treatment includes two stages, each with a treatment time of 8 min, a high-power radio frequency of 280 W, and a treatment temperature of 600 °C. After the previous stage is completed, there is a 5-minute pause and Ar gas is introduced for purging before proceeding to the next stage. S4. Under a third mixed gas atmosphere with H2 content of 4 vol% and the remainder being Ar, the third intermediate product is subjected to a fourth-stage plasma treatment to anneal the graphite and obtain the fourth intermediate product; wherein the radio frequency power is a medium-low power radio frequency of 140W, the treatment temperature is 550℃, and the treatment time is 15min. S5. The fourth intermediate product was ultrasonically dispersed in deionized water for 5 min, then centrifuged at 6500 rpm / min for 5 min. The ultrasonic washing and centrifugation were repeated 3 times. Finally, the product was vacuum dried at 0.1 MPa and 150℃ for 12 h to obtain high-purity, low-damage graphite powder.

[0048] Example 2: This embodiment provides a method for purifying graphite using a stepped plasma process, which is performed in accordance with the steps of Embodiment 1, with the only difference being: In step S0, the pickling conditions are changed to: soaking in a 3wt% HCl solution at room temperature for 3 hours; The remaining steps and parameters remain unchanged to obtain high-purity, low-damage graphite powder.

[0049] Example 3: This embodiment provides a method for purifying graphite using a stepped plasma process, which is performed in accordance with the steps of Embodiment 1, with the only difference being: In step S1, the radio frequency power of the first-stage plasma treatment is 80W, the treatment temperature is 150℃, the treatment time is 20min, and the protective gas is N2. The remaining steps and parameters remain unchanged to obtain high-purity, low-damage graphite powder.

[0050] Example 4: This embodiment provides a method for purifying graphite using a stepped plasma process, which is performed in accordance with the steps of Embodiment 1, with the only difference being: In step S1, the radio frequency power of the first-stage plasma treatment is 100W, the treatment temperature is 250℃, the treatment time is 10min, and the protective gas is He; The remaining steps and parameters remain unchanged to obtain high-purity, low-damage graphite powder.

[0051] Example 5: This embodiment provides a method for purifying graphite using a stepped plasma process, which is performed in accordance with the steps of Embodiment 1, with the only difference being: In step S2, the second plasma treatment is carried out in a first mixed gas atmosphere with an O2 content of 0.5 vol% and the remainder being N2, with a radio frequency power of 150 W, a treatment temperature of 300 °C, and a treatment time of 10 min. The remaining steps and parameters remain unchanged to obtain high-purity, low-damage graphite powder.

[0052] Example 6: This embodiment provides a method for purifying graphite using a stepped plasma process, which is performed in accordance with the steps of Embodiment 1, with the only difference being: In step S2, the second-stage plasma treatment is carried out in a first mixed gas atmosphere with an O2 content of 1.5 vol% and the remainder being He, with a radio frequency power of 180 W, a treatment temperature of 400 °C, and a treatment time of 5 min. The remaining steps and parameters remain unchanged to obtain high-purity, low-damage graphite powder.

[0053] Example 7: This embodiment provides a method for purifying graphite using a stepped plasma process, which is performed in accordance with the steps of Embodiment 1, with the only difference being: In step S3, the third-stage plasma treatment is carried out in a second mixed gas atmosphere with a trifluoromethane content of 0.5 vol% and the remainder being N2, with a radio frequency power of 300 W and a treatment temperature of 650 °C. The third-stage plasma treatment includes two stages, each with a treatment time of 5 min. After the previous stage is completed, there is a 3-minute pause and N2 is introduced for purging before proceeding to the next stage. The remaining steps and parameters remain unchanged to obtain high-purity, low-damage graphite powder.

[0054] Example 8: This embodiment provides a method for purifying graphite using a stepped plasma process, which is performed in accordance with the steps of Embodiment 1, with the only difference being: In step S3, the third-stage plasma treatment is carried out in a second mixed gas atmosphere with 0.1 vol% 1,1,1,2-tetrafluoroethane and the remainder being He. The radio frequency power is 250 W and the treatment temperature is 550 °C. The third-stage plasma treatment includes two stages, each with a treatment time of 10 min. After the previous stage is completed, there is an 8-minute pause and He is introduced for purging before proceeding to the next stage. The remaining steps and parameters remain unchanged to obtain high-purity, low-damage graphite powder.

[0055] Example 9: This embodiment provides a method for purifying graphite using a stepped plasma process, which is performed in accordance with the steps of Embodiment 1, with the only difference being: In step S4, the fourth plasma treatment is carried out in a third mixed gas atmosphere with H2 content of 3 vol% and the remainder being N2, with a radio frequency power of 130 W, a treatment temperature of 500 °C, and a treatment time of 20 min. The remaining steps and parameters remain unchanged to obtain high-purity, low-damage graphite powder.

[0056] Example 10: This embodiment provides a method for purifying graphite using a stepped plasma process, which is performed in accordance with the steps of Embodiment 1, with the only difference being: In step S4, the fourth plasma treatment is carried out in a third mixed gas atmosphere with H2 content of 5 vol% and the remainder being He, with a radio frequency power of 145 W, a treatment temperature of 600 °C, and a treatment time of 10 min. The remaining steps and parameters remain unchanged to obtain high-purity, low-damage graphite powder.

[0057] Example 11: This embodiment provides a method for purifying graphite using a stepped plasma process, which is performed in accordance with the steps of Embodiment 1, with the only difference being: In step S0, a natural graphite raw material with a fixed carbon content of 98% is selected as the purification target. The remaining steps and parameters remain unchanged to obtain high-purity, low-damage graphite powder.

[0058] Example 12: This embodiment provides a method for purifying graphite using a stepped plasma process, which is performed in accordance with the steps of Embodiment 1, with the only difference being: In step S0, the pickling conditions are changed to: soaking in a 0.5wt% HCl solution at room temperature for 12 hours; The remaining steps and parameters remain unchanged to obtain high-purity, low-damage graphite powder.

[0059] Example 13: This embodiment provides a method for purifying graphite using a stepped plasma process, which is performed in accordance with the steps of Embodiment 1, with the only difference being: In step S3, the third-stage plasma treatment process was not divided into two stages, but was instead performed continuously for 16 minutes. The remaining steps and parameters remain unchanged to obtain high-purity, low-damage graphite powder.

[0060] Example 14: This embodiment provides a method for purifying graphite using a stepped plasma process, which is performed in accordance with the steps of Embodiment 1, with the only difference being: In step S3, the third-stage plasma treatment includes two stages, each with a treatment time of 30 minutes. After the previous stage is completed, there is a 5-minute pause and Ar gas is introduced for purging before proceeding to the next stage. The remaining steps and parameters remain unchanged to obtain high-purity, low-damage graphite powder.

[0061] Comparative Example 1: This comparative example provides a method for purifying graphite powder, which is carried out in accordance with the steps of Example 1, with the only difference being: Step S1 is omitted, that is, the first-stage plasma treatment is not performed, and step S2 is performed directly after step S0 is completed; The remaining steps and parameters remain unchanged to obtain purified graphite powder.

[0062] Comparative Example 2: This comparative example provides a method for purifying graphite powder, which is carried out in accordance with the steps of Example 1, with the only difference being: Step S2 is omitted, that is, the second-stage plasma treatment is not performed, and step S3 is performed directly after step S1; The remaining steps and parameters remain unchanged to obtain purified graphite powder.

[0063] Comparative Example 3: This comparative example provides a method for purifying graphite powder, which is carried out in accordance with the steps of Example 1, with the only difference being: Step S3 is omitted, that is, the third-stage plasma treatment is not performed, and step S4 is performed directly after step S2 is completed. The remaining steps and parameters remain unchanged to obtain purified graphite powder.

[0064] Comparative Example 4: This comparative example provides a method for purifying graphite powder, which is carried out in accordance with the steps of Example 1, with the only difference being: Step S4 is omitted, that is, the fourth-stage plasma treatment is not performed, and step S5 is performed directly after step S3 is completed. The remaining steps and parameters remain unchanged to obtain purified graphite powder.

[0065] Comparative Example 5: This comparative example provides a method for purifying graphite powder, which is carried out in accordance with the steps of Example 1, with the only difference being: In step S1, the radio frequency power of the first-stage plasma treatment is changed to 40W and the treatment temperature is changed to 320℃. The remaining steps and parameters remain unchanged to obtain purified graphite powder.

[0066] Comparative Example 6: This comparative example provides a method for purifying graphite powder, which is carried out in accordance with the steps of Example 1, with the only difference being: In step S2, the radio frequency power of the second plasma treatment is changed to 120W and the treatment time is 35 min. The remaining steps and parameters remain unchanged to obtain purified graphite powder.

[0067] Comparative Example 7: This comparative example provides a method for purifying graphite powder, which is carried out in accordance with the steps of Example 1, with the only difference being: In step S3, the third-stage plasma treatment is carried out in a second mixed gas atmosphere with 5 vol% 1,1,1,2-tetrafluoroethane and the remainder being Ar. The remaining steps and parameters remain unchanged to obtain purified graphite powder.

[0068] Comparative Example 8: This comparative example provides a method for purifying graphite powder, which is carried out in accordance with the steps of Example 1, with the only difference being: In step S3, the second mixed gas atmosphere is replaced with a pure Ar atmosphere; The remaining steps and parameters remain unchanged to obtain purified graphite powder.

[0069] Comparative Example 9: This comparative example provides a method for purifying graphite powder, which is carried out in accordance with the steps of Example 1, with the only difference being: In step S4, the third mixed gas atmosphere is replaced with a pure Ar atmosphere; The remaining steps and parameters remain unchanged to obtain purified graphite powder.

[0070] Comparative Example 10: This comparative example provides a method for purifying graphite powder, which is carried out in accordance with the steps of Example 1, with the only difference being: In step S2, the O2 content in the first mixture is changed to 10 vol%, with the remainder being Ar; The remaining steps and parameters remain unchanged to obtain purified graphite powder.

[0071] Comparative Example 11: This comparative example provides a method for purifying graphite powder, which is carried out in accordance with the steps of Example 1, with the only difference being: In step S2, the processing time for the second plasma treatment is 1 minute; In step S3, the radio frequency power of the third plasma treatment is 200W; The remaining steps and parameters remain unchanged to obtain purified graphite powder.

[0072] Comparative Example 12: This comparative example provides a method for purifying graphite powder, which is carried out in accordance with the steps of Example 1, with the only difference being: In step S4, the H2 content in the third mixed gas atmosphere is changed to 0.5 vol%, with the remainder being Ar; The remaining steps and parameters remain unchanged to obtain purified graphite powder.

[0073] Test example: (1) The fixed carbon content of the graphite powder products purified in Examples 1-14 and Comparative Examples 1-12 was tested by box-type high-temperature furnace method. The test results are shown in Table 1. Table 1. Test results of fixed carbon content and ash content

[0074] (2) The interlayer spacing of the graphite raw material used in Example 1, and the graphite powder products purified in Examples 1-14 and Comparative Examples 1-12 were tested using an X-ray diffractometer. The specific steps are as follows: Taking the purified graphite powder product from Example 1 as an example, an appropriate amount of the graphite powder to be tested was weighed into a sample cell and pressed into a test piece with a smooth surface. Subsequently, the test piece was placed on the sample stage of an X-ray diffractometer, and Cu Kα radiation (λ=0.15418 nm) was used to scan within a 2θ range of 10°-90° with a step size of 0.02°. The resulting X-ray diffraction pattern is shown below. Figure 1 As shown; the remaining samples were subjected to the same testing procedures as in Example 1. The X-ray diffraction patterns of the samples from Comparative Example 1, Examples 2-4, and the graphite raw material used in Example 1 are shown below. Figure 1 , Figure 2 As shown; record the 2θ values ​​of the diffraction peak positions of the (002) crystal plane respectively, and substitute them into the Bragg equation (Ⅰ) below to calculate the interlayer spacing. The calculation results are recorded in Table 2. Equation (I); (3) The defect density of the graphite raw material used in Example 1, and the graphite powder products purified in Examples 1-14 and Comparative Examples 1-12 were tested using a Raman spectrometer. The specific steps are as follows: Taking the purified graphite powder product from Example 1 as an example, an appropriate amount of the graphite powder to be tested was weighed onto a glass slide and pressed into a thin sample with a smooth surface. Subsequently, the sample was placed on the sample stage of a Raman spectrometer, and a laser with a wavelength of 532 nm was used as the excitation source. The sample was then subjected to a Raman spectrometer at a wavelength of 100-3000 cm⁻¹. -1 Raman spectra were collected within the wavenumber range, and the resulting Raman spectra are shown below. Figure 3 As shown; the remaining samples were subjected to the same testing procedures as in Example 1. The X-ray diffraction patterns of the samples from Comparative Example 1, Examples 2-4, and the graphite raw material used in Example 1 are shown below. Figure 3 , Figure 4 As shown; record the intensities of peak D and peak G respectively, and calculate I. D / I G The ratios are recorded in Table 2; where I D / I G The smaller the ratio, the lower the defect density.

[0075] Table 2. Test results of graphite interlayer spacing and defect density

[0076] As shown in Tables 1 and 2, compared with the graphite powder purified in Comparative Examples 1-12, the high-purity, low-damage graphite powder purified in Examples 1-14 of this invention has the characteristics of high fixed carbon content and low ash content (besides fixed carbon content and ash content, the remainder is volatile matter), and its interlayer spacing is basically consistent with that of the graphite raw material, resulting in a low defect density. Furthermore, through further control of the purification process, the high-purity, low-damage graphite powder purified in Examples 1-11 has an even higher fixed carbon content (≥99.995%), lower ash content (<0.005%), and a lower defect density (I... D / I G (less than 0.13).

[0077] The above results are mainly attributed to the synergistic mechanism of the multiple-stage plasma treatment in this invention, which proceeds "from the surface to the core, from easy to difficult, and in a gentle and progressive manner." The specific mechanism of action is as follows: (1) First-stage plasma treatment (protective gas plasma physical sputtering): The graphite surface is gently bombarded by inert gas ions of low-power radio frequency plasma, which can physically desorb water, organic matter and light oxides without destroying the carbon skeleton. This provides a clean and activated surface for subsequent steps and avoids local over-reaction caused by impurities.

[0078] (2) Second-stage plasma treatment (selective plasma etching under the first mixed gas atmosphere): By precisely controlling the O2 concentration and radio frequency power, active oxygen free radicals can attack the defect sites on the graphite edge, thereby generating oxygen-containing functional groups within a controllable range, gently expanding the interlayer spacing and increasing the reactivity of interlayer impurities. This effectively avoids the problems of basal surface ablation and excessive interlayer spacing caused by conventional oxygen plasma.

[0079] (3) Third-stage plasma treatment (deep lattice impurity removal by plasma under a second mixed gas atmosphere): Low-concentration fluorocarbon gas is used in combination with high radio frequency power, which enables the fluorocarbon gas to dissociate and generate highly active F radicals, allowing them to penetrate into the graphite interlayer and lattice interior, react with difficult-to-remove metal impurities to generate low-boiling-point fluorides, which are then discharged with the gas flow. In addition, during the third-stage plasma treatment, the secondary deposition of non-volatile fluorides on the graphite surface can be effectively prevented by staged treatment combined with intermediate purging.

[0080] (4) Fourth-stage plasma treatment (plasma annealing and repair under the third mixed gas atmosphere): By using a mixed atmosphere with medium and low power and an appropriate amount of H2, active H radicals can react with residual dangling bonds such as CF and CO, converting them into volatile products for removal; at the same time, it promotes the removal of sp 2 Carbon layer rearrangement and stitching defects are achieved, thereby repairing minor damage that may be introduced by the aforementioned process.

[0081] In summary, this invention achieves low-damage purification of graphite by matching and coordinating the process parameters of four stages of plasma treatment. Relying on a progressive process approach that proceeds "from surface to core, from easy to difficult, and with gentle steps," the process sequentially involves four core stages: protective gas plasma surface desorption, first mixed gas plasma interlayer channel etching, second mixed gas plasma deep lattice impurity removal, and third mixed gas plasma annealing repair. Combined with pretreatment and post-treatment steps, this results in the final purification of natural graphite with a fixed carbon content of ≥99.88% (preferably ≥99.995%), ash content ≤0.11% (preferably <0.005%), and a significant reduction in defect density.

[0082] The above embodiments illustrate and describe the main features and advantages of the present invention in detail. However, the present invention is not limited to the embodiments described. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of the present invention. All such equivalent modifications or substitutions are included within the scope defined by the claims of the present invention.

Claims

1. A method for purifying graphite using a step-by-step plasma process, characterized in that, The method includes: S1. The graphite raw material is subjected to a first-stage plasma treatment to obtain a first intermediate product; the first-stage plasma treatment is carried out in a protective gas. S2. In the first mixed gas, the first intermediate product is subjected to a second-stage plasma treatment to obtain a second intermediate product; the first mixed gas contains 0.1 vol%-5 vol% of oxidizing gas, and the remainder is protective gas; the oxidizing gas includes one or more of O2, O3, and N2O. S3. In the second mixed gas, the second intermediate product is subjected to a third-stage plasma treatment to obtain a third intermediate product; the second mixed gas contains 0.1 vol%-1.5 vol% of fluorocarbon gas, with the remainder being protective gases; the fluorocarbon gas includes 1,1,1,2-tetrafluoroethane and / or trifluoromethane. S4. In the third mixed gas, the third intermediate product is subjected to a fourth-stage plasma treatment to obtain a fourth intermediate product; the third mixed gas contains 1 vol%-10 vol% of reducing gas, and the remainder is protective gas; the reducing gas includes one or more of H2, NH3, and CH4. S5. Wash the fourth intermediate product to obtain high-purity, low-damage graphite powder. The radio frequency power of the first-stage plasma treatment is 50-120 W; The radio frequency power of the second-stage plasma treatment is 150-230 W; The radio frequency power of the third-stage plasma treatment is 250-350 W; The radio frequency power of the fourth-stage plasma treatment is 120-150 W.

2. The method for purifying graphite using step-by-step plasma according to claim 1, characterized in that, The temperature of the first-stage plasma treatment is 100-500℃, and the time of the first-stage plasma treatment is 5-60 min.

3. The method for purifying graphite using step-by-step plasma according to claim 1, characterized in that, The temperature of the second-stage plasma treatment is 200-800℃, and the time of the second-stage plasma treatment is 2-30 min.

4. The method for purifying graphite using step-by-step plasma according to claim 1, characterized in that, The temperature of the third-stage plasma treatment is 500-1000℃.

5. The method for purifying graphite using step-by-step plasma according to claim 1 or 4, characterized in that, The third-stage plasma treatment includes at least two stages, each with a treatment time of 2-30 minutes. After the previous stage is completed, there is a 1-10 minute pause and a protective gas is introduced for purging before proceeding to the next stage.

6. The method for purifying graphite using step-by-step plasma according to claim 1, characterized in that, The temperature of the fourth plasma treatment is 400-800℃, and the time of the fourth plasma treatment is 5-60 min.

7. The method for purifying graphite using step-by-step plasma according to claim 1, characterized in that, Before step S1, there are also pretreatment steps of acid washing, water washing and drying of graphite raw materials.

8. The method for purifying graphite using step-by-step plasma according to claim 7, characterized in that, The graphite raw material was acid-washed using an HCl solution with a concentration of 0.5wt%-15wt%.

9. The method for purifying graphite using step-by-step plasma according to claim 1, 7, or 8, characterized in that, The graphite raw material is natural graphite with a fixed carbon content of 95%-99%.

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