Ultrahigh purity nanographite and method for preparing the same

Abstract

The application provides a kind of nano graphite and its preparation method, relate to the technical field of nano graphite.The nano graphite has the characteristics of high carbon content, few layer sheet structure and low metal impurity content.The preparation method comprises: graphite raw material is subjected to acid intercalation reaction to obtain graphite intercalation compound, and metal impurities are removed by electrochemical ion exchange purification, then few layer nano graphite dispersion liquid is obtained by ion exchange and ultrasonic stripping, and finally lattice defects are repaired by rapid heating graphitization annealing.The application realizes the deep removal of metal impurities and the efficient repair of lattice defects by combining the process of electrochemical ion exchange purification and rapid graphitization annealing, and effectively inhibits the sintering and growth of nano graphite sheets.

Description

Technical Field

[0001] This invention relates to the field of ultra-high purity nano-graphite technology, specifically to an ultra-high purity nano-graphite and its preparation method. Background Technology

[0002] Ultra-high purity nano-graphite (few-layer graphene) exhibits excellent electrical, thermal, mechanical, and chemical stability, showing broad application prospects in semiconductor heat dissipation materials, high-conductivity electrodes, lithium-ion battery anodes, electromagnetic shielding, and high-performance composite materials. Currently, the mainstream preparation methods for ultra-high purity nano-graphite include mechanical exfoliation, redox methods, and chemical intercalation exfoliation, but all have insurmountable technical drawbacks. Although mechanical exfoliation can produce high-quality few-layer graphite, its yield is extremely low and its cost is high, making it impossible to achieve large-scale production. The oxidation-reduction method involves intercalating graphite with a strong oxidant followed by reduction and exfoliation. This method is simple and has a high yield, but it introduces a large number of oxygen-containing functional groups and lattice defects, resulting in low graphitization and a significant decrease in electrical conductivity. Furthermore, subsequent high-temperature reduction is insufficient to completely repair these defects. Traditional chemical intercalation and exfoliation methods utilize strong acid or alkali metal intercalation followed by ultrasonic exfoliation. While this method can achieve a certain scale of production, it is difficult to deeply remove the residual metal cations (such as iron, nickel, copper, sodium, etc.) between the layers. In existing technologies, the total content of metal impurities in the products is usually higher than 10 ppm, and the content of some elements even exceeds 5 ppm. These impurities can act as charge carrier scattering centers and catalytic active sites, which seriously reduces the electrical properties and chemical stability of the materials.

[0003] In addition, high-temperature graphitization annealing is usually required to repair lattice defects. However, the traditional annealing process has a slow heating rate and a long holding time, which can easily lead to the sintering and growth of ultra-high purity nano-graphite sheets, destroying their nanostructure characteristics and causing the lateral sheet diameter to exceed 1μm, thus losing the unique advantages of nanomaterials. Summary of the Invention

[0004] The aforementioned drawbacks severely limit the application of ultra-high purity nanographite in high-end electronics, aerospace, and other fields with stringent requirements for purity and performance. Therefore, the technical problem this invention aims to solve is to provide an ultra-high purity nanographite and its preparation method, thereby simultaneously achieving deep removal of metal impurities, efficient repair of lattice defects, and effective suppression of nanographite sheet sintering.

[0005] An ultra-high purity nano-graphite, wherein the carbon content of the nano-graphite is 99.95% to 99.99%, the number of layers is 1 to 10, the transverse sheet diameter is 150 nm to 450 nm, the total content of metal impurities does not exceed 3 ppm, and the powder conductivity is not less than 500 S / m.

[0006] It also includes a method for preparing ultra-high purity nano-graphite, comprising the following steps: Step 1, Intercalation reaction: Graphite raw material with a carbon content of not less than 99% is mixed with concentrated sulfuric acid and concentrated nitric acid. The mass ratio of concentrated sulfuric acid to graphite raw material is 10:1 to 30:1, and the mass ratio of concentrated nitric acid to graphite raw material is 0.5:1 to 2:1. The mixture is stirred at 0 to 10°C for 12 to 24 hours, filtered, and washed to obtain the graphite intercalation compound. Step 2, electrochemical ion exchange purification: The graphite intercalation compound is pressed into a sheet as the working electrode, and a tetraethylammonium tetrafluoroborate propylene carbonate solution is used as the organic electrolyte. A platinum sheet is used as the counter electrode, and a silver wire quasi-reference electrode is used as the reference electrode. Cyclic voltammetry is performed for 30 to 100 cycles under the conditions of potential window -1.5V to +2.0V (relative to Ag / Ag⁺). During the scan, ultrasonic treatment is used to allow the metal impurity ions to migrate into the organic electrolyte under the action of electric field force and concentration gradient. Step 3, electrolyte removal: Wash the sheet sequentially with perfluorohexane 3 to 5 times, ethanol 2 to 3 times, and deionized water until neutral to remove residual organic electrolyte from the surface; Step 4, interlayer cation removal and ultrasonic exfoliation: The graphite intercalation compound is dispersed in dilute sulfuric acid with a concentration of 0.1 mol / L to 0.5 mol / L, and stirred at 40℃ to 60℃ for 1 h to 3 h for competitive ion exchange. After filtration and washing, it is dispersed in deionized water and ultrasonically treated at a power of 50W to 300W for 10 min to 60 min for interlayer exfoliation to obtain a few-layer ultra-high purity nano-graphite dispersion. Step 5, Separation, Collection and Drying: Centrifuge the dispersion to collect the precipitate, wash with deionized water, and vacuum dry at 80℃ for 8 to 12 hours; Step 6, graphitization annealing repair: Under inert gas protection, heat the dry powder to 1500℃ to 2000℃ at a heating rate of 10℃ / min to 20℃ / min, hold for 0.5h to 2h, then cool to below 300℃ at a cooling rate of 10℃ / min to 20℃ / min, and allow to cool naturally to room temperature. Beneficial effects

[0007] This process, combining electrochemical ion exchange purification with rapid graphitization annealing, solves the problems of deep removal of metal impurities and the difficulty in simultaneously achieving lattice repair and sintering prevention in existing technologies. Electrochemical ion exchange purification utilizes the insertion and extraction of organic cations under an electric field, effectively opening the interlayer channels in graphite. This allows metal impurity ions to migrate deeply into the electrolyte under the dual drive of electric field force and concentration gradient, achieving a low level of residual metal impurities that is difficult to achieve with traditional acid washing methods. Rapid heating graphitization annealing achieves efficient repair of lattice defects at sufficiently high temperatures, while controlling the heating / cooling rate and holding time effectively inhibits the sintering and growth of ultra-high purity nanographite sheets. The synergistic effect of these two methods yields ultra-high purity nanographite products that combine high purity, low defects, and stable nanostructure characteristics. Attached Figure Description

[0008] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0009] Figure 1 This is a process flow diagram of the present invention. Detailed Implementation

[0010] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0011] like Figure 1 The present invention discloses an ultra-high purity nano-graphite, wherein the carbon content of the nano-graphite is 99.95% to 99.99%, the number of layers is 1 to 10, the transverse sheet diameter is 150 nm to 450 nm, the total content of metal impurities is not more than 3 ppm, and the powder conductivity is not less than 500 S / m.

[0012] The above performance indicators were determined using conventional methods in the field: carbon content was determined by elemental analysis or X-ray photoelectron spectroscopy; the number of flakes was determined by atomic force microscopy; the transverse flake diameter was statistically determined by transmission electron microscopy; the total content of metal impurities was determined by inductively coupled plasma mass spectrometry; and the powder conductivity was determined by the four-probe method.

[0013] As an optional embodiment, the number of layers is 1 to 5, and the lateral wafer diameter is 200 nm to 400 nm.

[0014] The number of layers is controlled by the ultrasonic exfoliation conditions in step four, with the ultrasonic power and processing time determining the degree of exfoliation. After standing and layering, a few-layer ultra-high purity nano-graphite dispersion with uniform sheet size can be obtained. After electrochemical ion exchange purification in step two, the metal impurity content in the graphite intercalation compound can be reduced to below 1 ppm, laying the foundation for low metal impurity content in the final product.

[0015] As an optional embodiment, the metallic impurity is one or more of iron, nickel, copper, aluminum, sodium, potassium, calcium, and magnesium, and the total content of the metallic impurity does not exceed 1 ppm.

[0016] Step two, electrochemical ion exchange purification, is the core step that distinguishes this method from traditional acid washing purification. Tetraethylammonium cations are embedded in the graphite interlayer at a negative potential during cyclic voltammetry scanning, increasing the interlayer spacing and providing a spatial channel for the migration of metal impurity ions; at a positive potential, they are desorbed, carrying away the metal impurity ions that have migrated to the interlayer. After multiple cycles of scanning, the metal impurity removal efficiency is significantly improved. Ultrasonic treatment further promotes ion diffusion and transport within the interlayer. Step two and step four, with their competitive ion exchange using dilute sulfuric acid, form a synergistic purification system: step two achieves deep migration and removal of metal impurities through electrochemical driving force, while step four removes residual organic cations in the interlayer through competitive replacement with hydrogen ions. The combination of these two steps achieves a low level of residual metal impurities that is difficult to achieve with traditional processes.

[0017] As an optional embodiment, the interlayer spacing of the nanographite is 0.336 nm to 0.339 nm.

[0018] The interlayer spacing is determined by the graphitization annealing temperature in step six. The higher the annealing temperature and the longer the holding time, the closer the interlayer spacing is to the ideal graphite of 0.335 nm. Rapid heating combined with short-term holding effectively suppresses layer sintering while repairing lattice defects, thereby obtaining ultra-high purity nano-graphite products with high electrical conductivity.

[0019] As an optional embodiment, the graphite raw material in step one is natural flake graphite, expandable graphite, or high-purity pyrolytic graphite, with a particle size of 100 mesh to 500 mesh and a carbon content of 99% to 99.9%; the concentrated sulfuric acid has a mass fraction of 95% to 98%, the concentrated nitric acid has a mass fraction of 65% to 68%, and the mass ratio of concentrated nitric acid to graphite raw material is 0.8:1 to 1.5:1. The reaction is carried out at 0 to 10°C with stirring for 12 to 24 hours.

[0020] Concentrated sulfuric acid acts as the intercalation substrate, entering the graphite interlayer to form intercalation compounds. Concentrated nitric acid acts as an oxidizing intercalation aid, introducing polar groups at the edge of the sheets to promote sulfate ion insertion. Low-temperature conditions can maintain intercalation efficiency while inhibiting excessive oxidation, avoiding the introduction of too many oxygen-containing functional groups and structural defects.

[0021] As an optional embodiment, the sheet thickness in step two is 0.5 mm to 2 mm, and the compression density is 0.8 g / cm³ to 1.5 g / cm³; the concentration of the tetraethylammonium tetrafluoroborate is 0.8 mol / L to 1.2 mol / L; the scanning rate is 20 mV / s to 80 mV / s; and the power of the ultrasonic treatment is 20 W to 50 W, and the frequency is 40 kHz to 60 kHz.

[0022] A pressing density of 0.8 g / cm³ to 1.5 g / cm³ maintains the connectivity of ion transport channels, ensuring electrolyte penetration into the sheet. A potential window of -1.5 V to +2.0 V provides a safety margin to prevent decomposition of propylene carbonate solvent. The number of cycles and scan rate affect the adequacy of ion exchange; more than 30 cycles ensure efficient removal of metal impurities. Repeating the scan with fresh electrolyte can further improve the removal effect.

[0023] As an optional embodiment, step three involves sequentially washing with perfluorohexane 3 to 5 times, ethanol 2 to 3 times, and deionized water until neutral. Perfluorohexane has good solubility for both tetraethylammonium tetrafluoroborate and propylene carbonate, and is chemically inert, does not react with interlayer sulfuric acid, and can effectively remove residual organic electrolyte from the surface.

[0024] The three-stage washing process (perfluorohexane, ethanol, and water) effectively removes surface organic residues, preventing contamination of the product by electrolyte residues in subsequent steps. Perfluorohexane washing is a key step, as its ability to dissolve organic electrolytes is superior to that of conventional organic solvents.

[0025] As an optional embodiment, the concentration of dilute sulfuric acid in step four is 0.2 mol / L to 0.3 mol / L, and the reaction is carried out by stirring at 50°C to 60°C for 1.5 h to 2.5 h; the power of ultrasonic treatment is 100 W to 200 W, the time is 20 min to 40 min, and the temperature is controlled by an ice-water bath.

[0026] Hydrogen ions in dilute sulfuric acid compete with the residual tetraethylammonium cations in the interlayer for ion exchange. The higher interlayer binding affinity of hydrogen ions drives the organic cations to be replaced in the liquid phase, achieving a removal rate of over 90%. Using dilute sulfuric acid instead of hydrochloric acid avoids the high-temperature corrosion of the carbon skeleton by chloride ions during annealing. Ultrasonic treatment achieves interlayer separation after the interlayer binding force has been weakened by both the electrochemical treatment in step two and the ion exchange in step four.

[0027] As an optional embodiment, the centrifugation speed in step five is 5000 rpm to 6000 rpm, and the centrifugation time is 15 min to 20 min; the inert gas in step six is ​​argon, with a flow rate of 200 mL / min to 500 mL / min; the graphitization annealing temperature is 1600℃ to 2000℃, and the holding time is 0.5 h to 1.5 h.

[0028] Rapid heating at 10℃ / min to 20℃ / min, combined with short-term holding at 0.5h to 1.5h, achieves a balance between lattice repair and sintering inhibition: During the high-temperature stage, carbon atoms gain sufficient thermal activation energy for lattice rearrangement, promoting vacancy defect healing and ordered interlayer stacking; simultaneously, the total high-temperature exposure time is limited, insufficient to induce significant sintering growth of ultra-high purity nanographite sheets. Cooling at a rate of 10℃ / min to 20℃ / min shortens the residence time in the high-temperature range, further inhibiting sintering. Once the temperature drops below 300℃, carbon atom diffusion essentially ceases, allowing for natural cooling.

[0029] Centrifugal separation is based on the size and density differences between few-layer ultra-high purity nano-graphite and insufficiently exfoliated particles, and selective collection is achieved by controlling the centrifugation speed and time. Steps five and six are interconnected: the drying conditions in step five directly affect the initial state of annealing in step six; vacuum drying effectively removes residual moisture and volatile substances, preventing the formation of bubbles or localized oxidation during annealing.

[0030] The performance tests of all embodiments and comparative examples of this invention were conducted using conventional methods in the art: carbon content was determined by elemental analysis or X-ray photoelectron spectroscopy; the number of layers was determined by atomic force microscopy; the transverse sheet diameter was statistically determined by transmission electron microscopy; the total content of metal impurities was determined by inductively coupled plasma mass spectrometry; and the powder conductivity was determined by the four-probe method.

[0031] Example 1: Intercalation reaction: Take 10g of 100-mesh natural flake graphite (carbon content 99.0%), add 200g of 98% concentrated sulfuric acid and 10g of 68% concentrated nitric acid, place in a polytetrafluoroethylene reaction vessel, and stir continuously at 100 rpm for 24h at 0~5℃. After the reaction is completed, filter and wash with deionized water until the pH of the filtrate is 5~6 to obtain the graphite intercalation compound.

[0032] Electrochemical ion exchange purification: The above-mentioned graphite intercalation compound was pressed into a sheet with a thickness of 1 mm, an area of ​​5 cm², and a density of 1.0 g / cm³ as the working electrode. A 1.0 mol / L tetraethylammonium tetrafluoroborate solution in propylene carbonate was used as the organic electrolyte. A platinum sheet was used as the counter electrode, and a silver wire quasi-reference electrode was used as the reference electrode. Cyclic voltammetry was performed under the conditions of a potential window of -1.5 V to +2.0 V, a scan rate of 50 mV / s, and 50 cycles. During the scan, ion diffusion was assisted by 35 W, 50 kHz ultrasound. The probe was 2.5 cm away from the surface of the sheet.

[0033] Electrolyte removal: Remove the sheet from the electrode clamp and wash it 4 times with 8 times the amount of sheet material in perfluorohexane, stirring for 12 minutes each time; then wash it 3 times with 4 times the amount of sheet material in ethanol, stirring for 10 minutes each time; finally wash it with deionized water until the pH of the filtrate is 7.

[0034] Interlayer cation removal and ultrasonic exfoliation: The treated graphite intercalation compound was dispersed in 0.25 mol / L dilute sulfuric acid and stirred at 55 °C for 2 h. After filtration, it was washed with deionized water until sulfate ions were undetectable by ion chromatography and the pH value was 6-7. It was then dispersed in deionized water to prepare a 2 mg / mL dispersion and ultrasonicated at 150 W and 30 kHz for 30 min, with the temperature controlled at 20 °C in an ice-water bath. After ultrasonication, it was allowed to stand for 45 min, and the upper suspension was taken as a few-layer ultra-high purity nano-graphite dispersion.

[0035] Separation, collection and drying: The above dispersion was centrifuged at 5500 rpm for 18 min, the precipitate was collected, and washed with deionized water until tetraethylammonium cations were undetectable by ICP-MS (detection limit <0.01 ppb) and the pH value was 7. The precipitate was then dried at 80℃ and vacuum degree ≤100 Pa for 10 h.

[0036] Graphitization annealing repair: The dried powder is placed in a tube furnace and rapidly heated to 1800℃ at a rate of 15℃ / min under an argon flow rate of 350mL / min. The temperature is held for 1 hour, and then rapidly cooled to below 300℃ at a rate of 15℃ / min. The powder is then allowed to cool naturally to room temperature to obtain an ultra-high purity nano-graphite product.

[0037] Example 2: Intercalation reaction: Take 10g of 200-mesh natural flake graphite (carbon content 99.5%), add 250g of 98% concentrated sulfuric acid and 12g of 68% concentrated nitric acid, place in a polytetrafluoroethylene reaction vessel, and stir continuously at 150 rpm for 15h at 5~10℃. After the reaction is completed, filter and wash with deionized water until the pH of the filtrate is 5~6 to obtain the graphite intercalation compound.

[0038] Electrochemical ion exchange purification: The above-mentioned graphite intercalation compound was pressed into a sheet with a thickness of 0.8 mm, an area of ​​4 cm², and a density of 1.2 g / cm³ as the working electrode. A 1.1 mol / L tetraethylammonium tetrafluoroborate solution in propylene carbonate was used as the organic electrolyte. A platinum sheet was used as the counter electrode, and a silver wire quasi-reference electrode was used as the reference electrode. Cyclic voltammetry was performed under the conditions of a potential window of -1.5 V to +2.0 V, a scan rate of 40 mV / s, and 60 cycles. During the scan, ion diffusion was assisted by 30 W, 45 kHz ultrasound. The probe was 2 cm away from the surface of the sheet.

[0039] Electrolyte removal: Remove the sheet from the electrode clamp and wash it three times with 6 times the amount of sheet material in perfluorohexane, stirring for 10 minutes each time; then wash it twice with 3 times the amount of sheet material in ethanol, stirring for 8 minutes each time; finally wash it with deionized water until the pH of the filtrate is 7.

[0040] Interlayer cation removal and ultrasonic exfoliation: The treated graphite intercalation compound was dispersed in 0.2 mol / L dilute sulfuric acid and stirred at 50°C for 2 h. After filtration, it was washed with deionized water until sulfate ions were undetectable by ion chromatography and the pH value was 6-7. It was then dispersed in deionized water to prepare a 1.5 mg / mL dispersion and ultrasonicated at 120 W power and 25 kHz frequency for 25 min, with the temperature controlled at 15°C in an ice-water bath. After ultrasonication, it was allowed to stand for 50 min, and the upper suspension was taken as a few-layer ultra-high purity nano-graphite dispersion.

[0041] Separation, collection and drying: The above dispersion was centrifuged at 5000 rpm for 20 min, the precipitate was collected, and washed with deionized water until tetraethylammonium cations were undetectable by ICP-MS (detection limit <0.01 ppb) and the pH value was 7. The precipitate was then dried at 80℃ and vacuum degree ≤100 Pa for 9 h.

[0042] Graphitization annealing repair: The dried powder was placed in a tube furnace and rapidly heated to 1900℃ at a rate of 18℃ / min under an argon flow rate of 300mL / min. The temperature was held for 0.8h, and then rapidly cooled to below 300℃ at a rate of 18℃ / min. The powder was then allowed to cool naturally to room temperature to obtain an ultra-high purity nano-graphite product.

[0043] Example 3: Intercalation reaction: Take 10g of 300-mesh high-purity pyrolytic graphite (carbon content 99.9%), add 300g of 98% concentrated sulfuric acid and 8g of 68% concentrated nitric acid, place in a polytetrafluoroethylene reaction vessel, and stir continuously at 80 rpm for 20h at 0~5℃. After the reaction is completed, filter and wash with deionized water until the pH of the filtrate is 5~6 to obtain the graphite intercalation compound.

[0044] Electrochemical ion exchange purification: The above-mentioned graphite intercalation compound was pressed into a sheet with a thickness of 1.2 mm, an area of ​​6 cm², and a density of 0.9 g / cm³ as the working electrode. A 0.9 mol / L tetraethylammonium tetrafluoroborate solution in propylene carbonate was used as the organic electrolyte. A platinum sheet was used as the counter electrode, and a silver wire quasi-reference electrode was used as the reference electrode. Cyclic voltammetry was performed under the conditions of a potential window of -1.5 V to +2.0 V, a scan rate of 60 mV / s, and 80 cycles. After the scan, the electrolyte was replaced and the scan was repeated once. During the scan, ion diffusion was assisted by 40 W, 55 kHz ultrasound. The probe was 3 cm away from the surface of the sheet.

[0045] Electrolyte removal: Remove the sheet from the electrode clamp and wash it 5 times with 10 times the amount of sheet material in perfluorohexane, stirring for 15 minutes each time; then wash it 3 times with 5 times the amount of sheet material in ethanol, stirring for 12 minutes each time; finally wash it with deionized water until the pH of the filtrate is 7.

[0046] Interlayer cation removal and ultrasonic exfoliation: The treated graphite intercalation compound was dispersed in 0.3 mol / L dilute sulfuric acid and stirred at 60℃ for 1.5 h. After filtration, it was washed with deionized water until sulfate ions were undetectable by ion chromatography and the pH value was 6-7. It was then dispersed in deionized water to prepare a 3 mg / mL dispersion and ultrasonicated at 180 W and 35 kHz for 25 min, with the temperature controlled at 25℃ in an ice-water bath. After ultrasonication, it was allowed to stand for 30 min, and the upper suspension was taken as a few-layer ultra-high purity nano-graphite dispersion.

[0047] Separation, collection and drying: The above dispersion was centrifuged at 6000 rpm for 15 min, the precipitate was collected, and washed with deionized water until tetraethylammonium cations were undetectable by ICP-MS (detection limit <0.01 ppb) and the pH value was 7. The precipitate was then dried at 80℃ and vacuum degree ≤100 Pa for 12 h.

[0048] Graphitization annealing repair: The dried powder is placed in a tube furnace and rapidly heated to 2000℃ at a rate of 20℃ / min under an argon flow rate of 400mL / min. The temperature is held for 0.5h, and then rapidly cooled to below 300℃ at a rate of 20℃ / min. The powder is then allowed to cool naturally to room temperature to obtain an ultra-high purity nano-graphite product.

[0049] Comparative Example 2: Intercalation Reaction: 10g of 100-mesh natural flake graphite (99.0% carbon content) was added to 200g of 98% concentrated sulfuric acid and 10g of 68% concentrated nitric acid. The mixture was placed in a polytetrafluoroethylene reaction vessel and stirred continuously at 100 rpm for 24 hours at 0-5℃. After the reaction, the mixture was filtered and washed with deionized water until the pH of the filtrate was 5-6, yielding the graphite intercalation compound. Acid Washing Purification: The graphite intercalation compound was dispersed in 1mol / L hydrochloric acid and stirred at 60℃ for 4 hours. After filtration, the mixture was washed with deionized water until neutral. The acid washing operation was repeated 3 times. Ultrasonic Exfoliation: The acid-washed graphite intercalation compound was dispersed in deionized water to prepare a 2mg / mL dispersion. The dispersion was ultrasonically treated at 150W power and 30kHz frequency for 30 minutes to obtain a few-layer ultra-high purity nano-graphite dispersion. Separation and drying: The dispersion was centrifuged at 5500 rpm for 18 min, the precipitate was collected, washed with deionized water, and dried under vacuum at 80℃ for 10 h. Slow annealing: The dried powder was placed in a tube furnace and heated to 1800℃ at a rate of 5℃ / min under argon flow of 350 mL / min, held at that temperature for 2 h, and then naturally cooled to room temperature to obtain a comparative ultra-high purity nano-graphite product.

[0050] Comparative Example 1 (Redox Method): Ultrasonic exfoliation: Graphene oxide was dispersed in deionized water to prepare a dispersion of 1 mg / mL, and ultrasonically treated for 1 h at a power of 300 W and a frequency of 40 kHz to obtain a graphene oxide dispersion.

[0051] Chemical reduction: Add 5 mL of hydrazine hydrate to the above dispersion, stir at 95 °C for 6 h, filter, wash with deionized water until neutral, and vacuum dry at 80 °C for 12 h to obtain reduced graphene oxide powder.

[0052] High-temperature annealing: Reduced graphene oxide powder was placed in a tube furnace and heated to 1800℃ at a rate of 5℃ / min under an argon flow rate of 350mL / min. The temperature was held for 2 hours and then naturally cooled to room temperature to obtain a comparative ultra-high purity nano-graphite product.

[0053] Performance comparison results Table 1 Comparison of key performance indicators of the examples and comparative products. Carbon content (%) 99.97±0.01 99.98±0.01 99.99±0.01 99.90±0.02 99.82±0.02 Number of layers (layers) 3~8 1~5 2~6 3~10 5~15 Lateral flake diameter (nm) 150~450 200~400 180~420 200~500 300~800 Total content of metallic impurities (ppm) 2.3±0.2 1.5±0.1 0.8±0.1 6.5±0.3 12.6±0.5 Iron content (ppm) 0.7±0.1 0.5±0.1 0.2±0.05 2.1±0.2 4.1±0.3 Sodium content (ppm) 0.4±0.05 0.3±0.05 0.1±0.05 1.2±0.1 2.8±0.2 Interlayer spacing (nm) 0.338±0.001 0.337±0.001 0.336±0.001 0.340±0.001 0.342±0.001 Half-peak width of XRD002 (°) 0.16±0.01 0.13±0.01 0.11±0.01 0.22±0.01 0.32±0.02 Surface oxygen content (at%) 0.6±0.05 0.4±0.05 0.3±0.05 1.0±0.1 1.5±0.1 Powder conductivity (S / m) 620±20 750±20 880±20 380±20 210±20 As can be clearly seen from Table 1, the ultra-high purity nano-graphite prepared in the three embodiments of the present invention is significantly superior to the two comparative examples in terms of purity, controllability of sheet structure, and electrical properties. Compared with Comparative Example 1 (redox method), the product of the present invention has lower residual metal impurities, fewer lattice defects, and no obvious sintering during annealing. Compared with Comparative Example 2, the present invention uses electrochemical ion exchange purification instead of traditional acid washing, reducing the total content of metal impurities from 6.5 ppm to below 0.8 ppm. Rapid heating graphitization annealing instead of slow annealing effectively suppresses sheet sintering, resulting in better control of lateral sheet size and increasing powder conductivity from 380 S / m to 880 S / m. Example 3, using high-purity pyrolytic graphite as raw material and optimizing the electrochemical ion exchange and annealing processes, reduces the total content of metal impurities to 0.8 ppm and achieves a powder conductivity of 880 S / m, demonstrating the best overall performance.

[0054] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A type of ultra-high purity nano-graphite, characterized in that, The nanographite has a carbon content of 99.95% to 99.99%, a number of layers of 1 to 10, a lateral sheet diameter of 150 nm to 450 nm, a total metal impurity content of no more than 3 ppm, and a powder conductivity of no less than 500 S / m.

2. The ultra-high purity nano-graphite according to claim 1, characterized in that, The number of layers is 1 to 5, and the lateral diameter is 200 nm to 400 nm.

3. The ultra-high purity nano-graphite according to claim 1, characterized in that, The metallic impurities are one or more of iron, nickel, copper, aluminum, sodium, potassium, calcium, and magnesium, and the total content of the metallic impurities does not exceed 2 ppm.

4. The ultra-high purity nano-graphite according to claim 1, characterized in that, The interlayer spacing of the nanographite is 0.336 nm to 0.339 nm.

5. A method for preparing ultra-high purity nano-graphite, applicable to the ultra-high purity nano-graphite according to any one of claims 1 to 4, characterized in that, Includes the following steps: Step 1, Intercalation reaction: Graphite raw material with a carbon content of not less than 99% is mixed with concentrated sulfuric acid and concentrated nitric acid. The mass ratio of concentrated sulfuric acid to graphite raw material is 10:1 to 30:1, and the mass ratio of concentrated nitric acid to graphite raw material is 0.5:1 to 2:

1. The mixture is stirred at 0 to 10°C for 12 to 24 hours, filtered, and washed to obtain the graphite intercalation compound. Step 2, electrochemical ion exchange purification: The graphite intercalation compound is pressed into a sheet as the working electrode, and a tetraethylammonium tetrafluoroborate propylene carbonate solution is used as the organic electrolyte. A platinum sheet is used as the counter electrode, and a silver wire quasi-reference electrode is used as the reference electrode. Cyclic voltammetry is performed for 30 to 100 cycles under the conditions of potential window -1.5V to +2.0V (relative to Ag / Ag⁺). During the scan, ultrasonic treatment is used to allow the metal impurity ions to migrate into the organic electrolyte under the action of electric field force and concentration gradient. Step 3, electrolyte removal: Wash the sheet sequentially with perfluorohexane 3 to 5 times, ethanol 2 to 3 times, and deionized water until neutral to remove residual organic electrolyte from the surface; Step 4, interlayer cation removal and ultrasonic exfoliation: The graphite intercalation compound is dispersed in dilute sulfuric acid with a concentration of 0.1 mol / L to 0.5 mol / L, and stirred at 40℃ to 60℃ for 1 h to 3 h for competitive ion exchange. After filtration and washing, it is dispersed in deionized water and ultrasonically treated at a power of 50W to 300W for 10 min to 60 min for interlayer exfoliation to obtain a few-layer ultra-high purity nano-graphite dispersion. Step 5, Separation, Collection and Drying: Centrifuge the dispersion to collect the precipitate, wash with deionized water, and vacuum dry at 80℃ for 8 to 12 hours; Step 6, graphitization annealing repair: Under inert gas protection, heat the dry powder to 1500℃ to 2000℃ at a heating rate of 10℃ / min to 20℃ / min, hold for 0.5h to 2h, then cool to below 300℃ at a cooling rate of 10℃ / min to 20℃ / min, and allow to cool naturally to room temperature.

6. The method for preparing ultra-high purity nano-graphite according to claim 5, characterized in that, The graphite raw material mentioned in step one is natural flake graphite, expandable graphite, or high-purity pyrolytic graphite, with a particle size of 100 mesh to 500 mesh and a carbon content of 99% to 99.9%; the concentrated sulfuric acid has a mass fraction of 95% to 98%, the concentrated nitric acid has a mass fraction of 65% to 68%, and the mass ratio of concentrated nitric acid to graphite raw material is 0.8:1 to 1.5:

1.

7. The method for preparing ultra-high purity nano-graphite according to claim 5, characterized in that, In step two, the sheet thickness is 0.5 mm to 2 mm, and the compression density is 0.8 g / cm³ to 1.5 g / cm³; the concentration of the tetraethylammonium tetrafluoroborate is 0.8 mol / L to 1.2 mol / L. The scanning rate is 20mV / s to 80mV / s; the power of the ultrasonic treatment is 20W to 50W, and the frequency is 40kHz to 60kHz.

8. The method for preparing ultra-high purity nano-graphite according to claim 5, characterized in that, In step four, the concentration of the dilute sulfuric acid is 0.2 mol / L to 0.3 mol / L, and the reaction is carried out at 50°C to 60°C with stirring for 1.5 h to 2.5 h. The ultrasonic treatment has a power of 100W to 200W and a duration of 20min to 40min.

9. The method for preparing ultra-high purity nano-graphite according to claim 5, characterized in that, The centrifugation speed in step five is 5000 rpm to 6000 rpm, and the centrifugation time is 15 min to 20 min; The inert gas mentioned in step six is ​​argon, with a flow rate of 200 mL / min to 500 mL / min, and the graphitization annealing temperature is 1600℃ to 2000℃, with a holding time of 0.5 h to 1.5 h.

10. The method for preparing ultra-high purity nano-graphite according to claim 5, characterized in that, After the cyclic voltammetric scan described in step two is completed, the electrolyte is replaced and the scan is repeated 1 to 2 times.

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