RU2845025C1 - Method of producing nanoscale graphite flakes with controlled degree of oxidation by electrochemical graphite exfoliation - Google Patents

Abstract

FIELD: chemistry; nanotechnology.

SUBSTANCE: method of producing nanoscale graphite flakes with a controlled degree of oxidation by electrochemical graphite exfoliation comprises carrying out the process in an electrochemical cell, containing working electrodes made of graphite, pre-treated by heat treatment, in air, and cathodes-counter electrodes made of stainless steel, in aqueous solutions of electrolytes, which are not protonic acids containing ions H⁺, NH₄ ⁺SO₄ ^-2, OH^-1, at temperature of not more than 90 °C at cyclic alternating polarity, varying potential from 0 to 30 V and constant removal of exfoliated particles.

EFFECT: invention provides a wide range of nano-graphite doping with oxygen from 1.6 to 29 at.% in a universal mode and simple cheap implementation of the process, low defectiveness of exfoliated nano-graphite, enlarging the range of products; low cost of production due to cheap widely available raw material and simplicity of technological implementation.

7 cl, 10 dwg

Description

Field of technology

The invention relates to the chemical industry, electrolytic methods for producing carbon, in particular a method for producing nanocarbon particles by splitting graphite, producing and processing nanostructures as discrete objects, and manufacturing and processing nanostructures. The invention concerns a method for producing nanosized particles (plates) of graphite by anodic electrochemical exfoliation of graphite in a potentiostatic mode of 10 V in a 0.1 M - 3.0 M aqueous solution of ammonium sulfate electrolyte in the presence of substances regulating the oxygen content from 1.6 to 29 at.% in exfoliated graphite products. A feature of the exfoliation process is the simplicity of the technological design of a two-electrode electrolyzer, and an expansion of the range of nanocarbon with various required oxygen alloying values. There are 11 examples.

Definitions of key terms used in disclosing the invention

The basis for obtaining nano-sized graphite plates is the process of exfoliation of natural or artificial graphite under the influence of physical, chemical or physicochemical action, called exfoliation. The description uses the following terms, which, although generally accepted by specialists in this field of technology, however require clarification in the context of the claimed invention in accordance with the universal terminology for describing graphene and graphene-containing materials:

Graphene is a hexagonally ordered atomic layer consisting of carbon atoms in sp2 hybridization; it can exist as a sol, be in a free (suspended) state or on a substrate. Graphene derivatives from the 2D material family cannot be called "graphene" and have their own unique name. Two-, three-layer graphene is a 2D material consisting of two or three layers connected to each other either by the AB type of a regular hexagonal close-packed lattice (HCCL), or chaotically rotated relative to each other.

Multilayer graphene is a 2D material (existing in a suspended state and on a substrate) containing from two to ten separate layers linked together along an extended plane by the ABA (regular GPUR) type, or by weak links with layers randomly rotated relative to each other.

Exfoliated graphite is a material obtained by partial exfoliation (thermally, chemically or mechanically) of graphite into multilayer particles (up to 100 nm thick) in which the π-π interaction is similar to graphite. Bianco A., Cheng H.-M., Enoki T. et al. All in the graphene family - A recommended nomenclature for two-dimensional carbon materials // Carbon. 2013. Vol. 65, No. 0. Pp. 1-6. The use of mechanical exfoliation methods for graphite allows one to obtain graphene films with a size of about 10 µm in plan, the yield of single-layer graphene is negligible (Meier J.C., Geim A.K. et al. On the roughness of single- and bi-layer grapheme membranes. Solid State Communications, 143, 2007, 101-109, Dideykin A.T. et al. Free graphene films from thermally expanded graphite. - J. Tech. Phys. 2010, v.80, pp.146-149.

Natural graphite. Graphite occurs naturally and can be found as stacks of crystalline flake-like forms that include anywhere from tens to thousands or more layers. These layers are typically arranged in an ordered RU 2 787 431 C1 publ. 2023-01-09 sequence, namely according to the so-called "ABAB stacking", where half of the atoms of each layer lie exactly above or below the center of a ring of six atoms in immediately adjacent layers. Because graphite flakes are "thick", their physical properties differ little from those of bulk graphite and are far from those of graphene. For example, graphite flakes are very weak in shear (i.e., the layers can be separated mechanically) and have highly anisotropic electronic, acoustic, and thermal properties. Because of the electronic interactions between adjacent layers, the electrical and thermal conductivity of graphite is lower than that of graphene. The specific surface area is also much lower, as would be expected from a material with a less flat geometry. In addition, at typical flake thicknesses, graphite is opaque to electromagnetic radiation at many different wavelengths. In some cases, graphite flakes can be macroscopic in size (e.g., 1 cm in length).

Chemical exfoliation of thermally expanded graphite involves treatment with strong acids to destroy interlayer bonds in graphite, followed by rapid heating with microwaves, for example (Liu Ch., Hu G., Gao H. Preparation of a few-layer and single-layer grapheme by exfoliation of expandable graphite in supercritical N.N-dimethylformamide.// J. of Supercritical Fluids, 63, 2012, 99-104, Hemandez Y. et al. High-yield production of grapheme by liquid phase exfoliation of graphite. - Nature Nanotech., 2008, v.2, p.563-568. The disadvantage of this highly productive method for synthesizing nanographite is the destruction of the graphene crystal lattice, which leads to a significant deterioration in its electrical conductivity.

Examples of liquid-phase exfoliation methods can be found in the publications of K. R. Paton, E. Varrla, C. Backes et al. "Scalable production of large quantities of defect-free few-layer graphene by shear exfoliation in liquids" // Nature Materials 13, p. 624-630 (2014) and Y. Hernandez, V. Nicolosi, M. Lotya "High-yield production of graphene by liquid-phase exfoliation of graphite" // Nature Nanotechnology 3, p. 563-568 (2008). A disadvantage of the liquid-phase exfoliation method is that although researchers who work with liquid-phase exfoliation methods often refer to the exfoliated carbon-containing flakes as "graphene", the thickness of the vast majority of flakes obtained by such exfoliation methods often exceeds 10 layers. Properties such as specific surface area also deteriorate with increasing scale thickness.

Basal plane of graphene: a single-layer layer of carbon atoms arranged in a two-dimensional cellular lattice. On the basal surface and edges of graphene there are inclusions - functional oxygen groups such as hydroxyl, oxyirane and carboxyl, which, from the point of view of the quantum-chemical nature of graphene, are defects that disrupt the crystal structure of the graphene lattice.

State of the art

Nanosized graphite plates, which are used to create a very wide range of products: to create conductive polymer composites, electronic ink, flexible conductive polymer cables in microelectronics, graphene memristors in nanoelectronics, composite materials in aircraft construction, shielding of microwave gigahertz and terahertz radiation, with high characteristics of thermal and heat resistance, resistance to thermal-oxidative destruction, effective anti-corrosion coatings of metals, high physical and mechanical characteristics, impact strength for the manufacture of bulletproof vests, plastic lubricants, the manufacture of thermoelectric generators of electricity, water purification from radiation pollution, oil and other organic and inorganic impurities, desalination of sea water using solar energy, in fuel cells and batteries, hydrogen energy, strengthening concrete and in other areas.

Such a wide variety of applications of nanosized graphite requires obtaining a material with a different set of physical and chemical properties in a wide range of specific quality indicators such as: electrical conductivity, temperature and thermal conductivity, mobility of electric charge carriers, specific surface, defectiveness, dispersibility in organic (polymer) material of various nature of hydrophilicity and hydrophobicity and others are mostly determined by the physical and chemical structure of nanographite and its surface. In addition to the size of the pack (the number of graphite layers in the pack) of the defect-free crystallite size in plan and thickness, a very important indicator is the decoration of the surface with various functional groups in this invention oxygen.

Analog. A method for exfoliating a layered material is known, which includes dispersing graphite in a liquid medium containing a surfactant, exposing said suspension or suspension to ultrasound at an energy level for a period of time sufficient to obtain separated nanoscale flakes Weigang Ma, Yingjun Liu, Shen Yan at all "Chemically doped macroscopic graphene fibers with significantly enhanced thermoelectric properties" //Nano Research 2018, 11(2): 741-750, doi.org/10.1007/s12274-017-1683-3.

Analogue. A method for producing graphene material is known Melezhik AV, Makarova LV, Konoplya MM, Bakai EA, Chuiko AA, Pikalov VK Method for producing microflake graphite. USSR A.S. No. 1566659 A1, C01B 31/04, 1988 (for official use, not published). According to this method, graphene material was obtained in the form of flakes with a thickness of 0.01-0.1 μm (= 10-100 nm) and a transverse size of 10-50 μm. In the description of this method, the resulting material was called "microflake graphite", since in 1988 the term "graphene" as a name for a material was not yet used, and graphene materials were unknown. In fact, "microflake graphite" was multilayer graphene. According to this method, crystalline (natural) graphite was treated with a solution of peroxodisulfuric acid or ammonium peroxodisulfate (persulfate) in anhydrous sulfuric acid. Anhydrous sulfuric acid was prepared by fixing concentrated sulfuric acid with oleum. When graphite is treated with a solution of peroxosulfuric acids or ammonium persulfate in anhydrous sulfuric acid, intercalated graphite compounds are formed, containing peroxosulfuric acid anions embedded between the carbon layers. Under normal conditions, these compounds gradually decompose, and an expanded intercalated graphite compound of a yellow-brown color (green at the initial stages of expansion) is formed, the apparent volume of which is 100-200 times greater than the bulk volume of the original graphite. This substance consists of nanosized flakes of the graphite compound, which are separated during mechanical dispersion of the material under conditions of shear deformation in the gap between the cylindrical rotor and the stator of the dispersing device. Next, hydrolysis of the dispersed material is carried out and the product is washed with water until the acid is removed. "Microflake graphite" is obtained, or, in modern terminology, multilayer graphene. This method was later described in a scientific article. Melezhik A.V., Rudy R.B., Makarova L.V., Chuiko A.A. Synthesis and properties of self-binding microflake graphite // Journal of Applied Chemistry, 1995, v. 68, issue 1, pp. 54-57 and in a number of other publications for 1991-1995. The common essential features of the considered and claimed methods are the treatment of crystalline graphite with a solution of peroxide compounds in anhydrous sulfuric acid, expansion of the obtained graphite compound at a low temperature (20-100 °C, as opposed to 900-1100 °C when obtaining thermally expanded graphite), dispersion of the expanded graphite material, hydrolysis and washing of the graphite material with water. Another version of such technology is the method for producing graphene nanoplatelets described in the works of Melezhyk A.V., Tkachev A.G. / Synthesis of graphene nanoplatelets from peroxosulfate graphite intercalation compounds // Nanosystems: Physics, Chemistry, Mathematics. 2014. Vol. 5. No. 2. P. 294-306 and [16]. Melezhyk A.V., Kotov V.A, Tkachev A.G. Optical Properties and Aggregation of Graphene Nanoplatelets // Journal of Nanoscience and Nanotechnology. 2016. Vol. 16.No. 1. P. 1067-1075.

Analogue. The method described in patent RU 2657504 published on 14.06.2018 is a development of the method described earlier in the works of these authors. The first stages - intercalation of graphite with a solution of ammonium persulfate in anhydrous sulfuric acid and low-temperature expansion - are the same. The resulting expanded reaction mass is then treated with water and washed on a filter until the sulfuric acid is removed. The disadvantages of the considered method are as follows: a solution of peroxosulfuric acids in sulfuric acid is obtained by mixing sulfuric acid, concentrated hydrogen peroxide and oleum. This process is accompanied by the release of a large amount of heat and is difficult to control when scaling up. Concentrated hydrogen peroxide is dangerous and unstable during storage. If even small impurities of compounds of catalytically active metals (iron, manganese, silver, etc.) get into it, a violent exothermic decomposition reaction can occur with the release of a large amount of gaseous oxygen. The reaction of graphite intercalation with a solution of peroxosulfuric acids in sulfuric acid is also exothermic and difficult to control on an industrial scale. The process is environmentally toxic and "dirty" with a large amount of waste.

Analog. The Hammer method is primarily used to produce highly oxygen-doped nanocarbon - graphene oxide. Recall that in the proposed invention, the oxygen content can reach 29 at.%, which is possible only by the Hammer method in graphene oxide. Chemical or electrochemical oxidation of graphite to graphite oxide with subsequent exfoliation can be used to produce graphene oxide flakes. One of the more commonly accepted approaches was first described by Hammer et al. in 1958 and is commonly referred to as the "Hammer method" J. Am. Chem. Soc. 80 (6), p. 1339-1339 (1958). In some cases, graphene oxide can subsequently be partially reduced with the removal of some of the oxygen. However, oxidative etching of graphite not only separates the graphene layers from each other, but also affects the hexagonal lattice of graphene. As a consequence, such graphene oxide is rich in defects and has low electrical and thermal conductivity and elastic modulus. In addition, planar etching of graphene flakes usually reduces the lateral dimensions of the flakes to several micrometers. Examples of graphite reduction methods can be found in J. Am. Chem. Soc. 134, p. 18689-18694 (2012) and [8] Angew. Chem. Int. Ed. 52, p. 754-757 (2013). Lithium and other reducing agents that can be used to reduce graphene from graphene oxide are excessively active and, therefore, complex in technology and problematic for disposal. The disadvantage of the method for synthesizing graphene oxide using the Hammer method is that graphite oxide is a very expensive substance due to the fact that the process of its production requires a large consumption of oxidizers and becomes explosive when scaled up. This greatly increases the cost of the resulting nanographite product graphene. It should be noted that the Hammer method is not universal and does not allow obtaining low-oxygen-alloyed nanocarbon. Table 2 presents the data for sample 11 of graphite oxidized under the conditions of the Hammer method for comparison with the method of the proposed invention in the presence of ammonium vanadate samples 9, 10.

Electrochemical exfoliation methods

Today, electrochemical exfoliation is considered as a promising strategy for the production of nanographite flakes on an industrial scale, environmentally friendly, with high efficiency and low cost. The essence of the method is the diffusion of ions from the solution into the interlayer space of graphite layers under the action of an electric field. Both cathode and anodic potentials are capable of promoting "guest" ions, which subsequently promote structural deformations in graphite working electrodes. Cationic intercalation mainly occurs in an organic environment and does not allow the process to be carried out under oxidizing conditions, and therefore, the final product graphene does not include oxygen groups. In most cases, the resulting product is nanographite and / or multigraphene with several layers, which require long-term ultrasonic or microwave processing to separate the layers.

Analogue. A method for the electrochemical production of nanosized graphite plates is known in RU 2763535 published on 30.12.2021, where an aqueous solution or melt of a mixed alkaline electrolyte containing at least two types of cations with different ionic radii is used. Direct, pulsed or alternating current is supplied to the electrodes. Electrodes made of graphite foil of the "graflex" brand are immersed in the electrolyte consisting of a mixture of 20 vol.% 0.5 M LiOH and 80 vol.% 0.5 M KOH. The temperature in the electrolyzer is maintained at 70 ° C. A pulsed current is supplied to the electrodes, with a cathode and anode pulse duration of 30 s, while the voltage on the electrodes is maintained at 10 V. The synthesis is carried out for 45 minutes. The suspension obtained after electrolysis is passed through a layer of ion-exchange resin KU-2-8-CHS to clean it from electrolyte and centrifuged for 5 minutes at 4000 rpm to separate large particles. The fugate is a suspension of nanosized graphite particles in water. Transmission electron microscopy methods have shown that nanosized graphite particles contain no more than 5 graphene layers, with lateral dimensions reaching 5 μm.

Analogue. An alternative to inorganic electrolytes are ionic liquids. Thus, the authors of the work [Liu N., Luo F, Wu H., Liu Y., Zhang C., Chen J. One-step ionic-liquid-assisted electrochemical synthesis of ionic-liquid-functionalized graphene sheets directly from graphite. Advanced. Functional. Materials, 2008, v. 18, p. 1518-1525] used a mixture of 10 ml of ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate) with 10 ml of water as an electrolyte. Two graphite rods were immersed in the solution at a distance of 6 cm from each other, connected to a DC source with a voltage of 15 V. After 30 minutes of the reaction, the anode began to deteriorate, and a black sediment formed at the bottom. The obtained material was collected, washed in absolute ethanol and dried at 60°C. The average length of the plates was 700 nm, the width was about 500 nm. The average thickness of the graphite nanoplate was about 1.1 nm. The disadvantage of this method of production is the difficulty of transition to industrial implementation, due to the high cost of ionic liquids and the complexity of their production, compared to solutions of inorganic electrolytes.

Analogue. Known KR 20170064908 published 2017.02.28 "Method for obtaining high-quality graphene flakes by electrochemical method, process of exfoliation and dispersion of graphene flake solution". The process of anodic exfoliation of graphite electrode is carried out in an aqueous-organic solution of inorganic electrolyte based on sulfate, sulfone, phosphate, phosphonium, nitrate and nitrite, carboxylate and carbon salts in concentrations of 0.001-3.0 M. The following compounds are used as solvents: water, ethanol, methanol, isopropanol, tetrahydrofuran (tetrahydrofuran: THF), benzene, xylene, toluene, cyclohexane, methylpyrrolidone (N-methyl 2-pyrrolidinone: NMP), dimethylformamide, dimethyl sulfoxide, dimethoxyethane (1,2-dimethoxyethane: DME), dichloromethane (1,2-dichloroethane), methylene chloride, chlorobenzene, chloroform, ethyl acetate, ethylene glycol, heptane, butanol (1-butanol, 2-butanol), butanone (2-butanone), acetonitrile, acetone, acetic acid, etc. They can be used alone or in combination. The voltage on the electrodes is constant, and the working voltage is in the range of 20 V to 100 mV, and then the current value can be 5 A at 1 mV. The graphene flake dispersion solution includes a solvent and a graphene flake in which it is dispersed in a solvent, and an anionic organic single molecule is combined on the surface of the graphene flake by non-separable functionalization (covalent bond). The additional anionic molecule is an alkyl or alkylene chain of 2 to 10 carbon atoms. Preferably, the anionic organic single molecule has a substituted benzene structure. The functional group of the anion is sulfinyl group (sulfinyl: SO ₂ ^-1 ), sulfonate group (sulfonate: SO ₃ ^-1 ), nitrate NO ₂ ^-1 (nitrite: NO ^-1 ), phosphite group (phosphite: PO ₃ ^-3 ), phosphate group (phosphate: PO ₄ ^-3 ), carboxylate group (carboxylate: CO ₂ ^-1 ), carbonate group (carbonate: CO ₃ ^-1 ). The anionic organic molecule having the structure of a benzene derivative is firmly bound via a pi bond to the graphene surface for greater efficiency of separation of graphene layers. More than 100 types of molecules are used as an organic molecule for covalent grafting onto the surface. As is clear from the description of the analogue, the main task of the process is exfoliation with simultaneous modification of the basal surface of graphene to stabilize the split off scales in the solution, preventing their subsequent coagulation. The analogue has the following disadvantages: a complex and very expensive exfoliation environment, a wide and non-specific stress potential, no specific oxygen content in graphene, no characteristics of the process, potentiostatic or galvanostatic, on which the nature of the resulting product depends.

Prototype. Known patent US 20180072573 A1 published 03.15.2018 "Graphene production". The authors of the patent propose to carry out the process in an electrochemical cell containing one or more electrodes insulated by a membrane or a permeable bag of anodes (working electrodes) made of natural or synthetic or pyrolytic graphite pre-treated with heat, plasma, electrochemical, ultrasonic treatment, in air or in a vacuum, one or more cathodes (counter electrodes) made of titanium metal, platinum, stainless steel, aluminum, graphite, or conductive glassy carbon, in aqueous or non-aqueous solutions of electrolytes that are not protic acids, containing ions Li ⁺ , Na ⁺ , K ⁺ , NH ₄ ⁺ SO ₄ ^-2 Cl ^-1 , NO ₃ ^-1 _, ClO ₄ ^-1 OH ^-1 individually or in a mixture at a temperature of less than 90°C with cyclic alternating polarity, changing in magnitude potential from 0 to 30 V, with constant removal of exfoliated particles. Carrying out exfoliation under the conditions of examples 5-15 of the patent, the exfoliation conditions are as follows: anode - commercially available graphite, cathode - titanium, potentiostatic mode 10 V, time from 24 to 6 hours, electrolyte concentration 0.001-1 M ammonium sulfate. Separation of the suspension from the solution was carried out by decantation, centrifugation, filtration. In examples 16-21 of the prototype patent, mixtures of inorganic cations such as Li ⁺ , Na ⁺ , K ⁺ , NH ₄ ⁺ are used, and anions such as SO ₄ ^-2 Cl ^- , NO ₃ ^- _, ClO ₄ ^- OH ^- of different ionic radius and chemical nature, which increases the rate of the exfoliation process. In example 22 of the prototype patent, the antioxidant TEMPO is additionally used. The analysis of exfoliation products is carried out on the basis of X-ray phase analysis by the presence of diffraction pattern bands in the region of angles 2θ ~ 25° and the appearance of a wide band in the region of 2θ ~ 12°, characterizing the presence of graphite oxide in the sample. The ratio of peak intensities (more or less) I _G / I _D and I _2D / I _G in Raman spectra is a measure of the characteristic of the defectiveness of exfoliated graphite. Thermogravimetric analysis is used during annealing of samples in an inert atmosphere with further comparison of mass loss more or less. As we have already indicated earlier, in the presented patent for graphene synthesis there is no evidence and methods characterizing the exfoliation product as graphene. Only indirect signs are used. Quantitative characteristics of the content of oxygen and other impurities are not provided. It would not be a mistake to characterize the exfoliation product with a more precise term in the classification of graphene-like materials, such as nanographite or nano-sized graphite flakes, so we will not use the term graphene in the invention formula. The disadvantages of the prototype are discussed below.

Disclosure of the essence of the invention

The technical result consists of a wide range of nanographite alloying with oxygen from 1.6 to 29 at.% in a universal mode and simple inexpensive hardware design of the process, low defectiveness of exfoliated nanographite, expansion of the range of manufactured products, low cost of products due to cheap widely available raw materials and simplicity of technological design.

Brief description of the drawings

The method of obtaining nano-sized graphite flakes with a controlled degree of oxidation by the method of electrochemical exfoliation of graphite is explained by photographs, diagrams, tables and graphs shown in Fig. 1-10.

Fig. 1. Scheme of the processes of electrochemical exfoliation of graphite in an aqueous solution of ammonium sulfate electrolyte.

Fig. 2. Scheme of reactions of oxidation of the carbon skeleton of graphite in the process of electrochemical exfoliation.

Fig. 3. DMSO consumption during anodic electrochemical exfoliation of graphite. 0.2 M aqueous solution of ammonium sulfate potentiostatic mode 10 V UV spectrum of 0.08 % DMSO in exfoliation electrolyte solution. 1 UV reaction time 0 s. 2 UV spectrum after 202 s. 3 UV spectrum after 752 s.

Fig. 4. Schematic diagram of the mechanism of hydroxyl radical inhibition by dimethyl sulfoxide.

Fig. 5. Formulas of additives of compounds used in the process of electrochemical exfoliation of graphite under standard conditions of a two-electrode cell in an aqueous solution of ammonium sulfate in potentiostatic mode at 10 V.

Fig. 6. Electron microscopic image of the edge of an exfoliated graphite flake.

Fig. 7. Elemental analysis of the initial Graflex foil using a Zeiss Supra-40 high-resolution scanning electron microscope with an INCA Energy X-ray energy dispersive analyzer. Full scale 6803 imp. Cursor: 1. 980 (45 imp.) keV. Left peak is carbon, right peak is oxygen.

Fig. 8. Diffraction patterns of the original foil (top) and exfoliated graphite (bottom).

Fig. 9. Raman spectrum of sample 10. From left to right: peak D, G, 2D.

Fig. 10. Results of the analysis of graphite electroexfoliation products using the method proposed in the present invention. *Initial foil, ** typical method of example 1 (cyclic cathode and anode pulse for 30 sec, 10 V, duty cycle 50%) for comparison, *** 2,2,6,6 - tetramethylpiperidine 1oxyl (TANAN = TEMPO), **** Hammer method.

Implementation of the invention

The method for producing nano-sized graphite flakes with a controlled oxidation degree by electrochemical exfoliation of graphite is based on the process of exfoliation of natural or artificial graphite under the action of a constant electric field in a potentiostatic mode at 10 V in an aqueous solution of ammonium sulfate and in the presence of some additional substances that regulate the oxidation degree of graphite carbon. The conditions for carrying out exfoliation, the concentration of electrolyte and additives, the duration of the process allow for a significant range of control over the oxygen content from 1.6 to 28.9 atomic percent, and control over the degree of defectiveness of the obtained flakes. Moreover, the main process parameters of the process - polarity, potential, main electrolyte, technological equipment (electrodes, electrolyzer cell design) - remain unchanged.

One of the most important issues in graphite exfoliation technology is the composition and quality of graphite exfoliation products, the analysis of which is not a trivial task. Below are presented the methods for analyzing graphite exfoliation products used in this application for an invention for each sample obtained.

Methods for analyzing nanographite samples obtained in this proposed invention.

1. Determination of the thickness of the exfoliated graphite layer using scanning electron microscopy.

Fig. 6 shows a typical surface of exfoliated graphite. It is a folded surface of randomly arranged graphite flakes, with the edges tending to twist. Measuring the thickness of a graphite stack is only possible at the end of the edge, which is quite difficult to do in practice. Fig. 7 shows a photograph of the edge of an exfoliated graphite flake, the thickness of which varies from 8 to 16 nm. To obtain a statistically reliable result, a significant array of data is required, which is extremely labor-intensive to obtain. Therefore, to estimate the thickness of the plates of a certain number of atomic layers, we used a method based on the analysis of X-ray diffraction of exfoliated graphite powder, where the analysis is averaged over hundreds of thousands of measurements and is statistically reliable.

2. Methodology for determining the number of graphite layers using X-ray phase analysis.

Phase analysis of the exfoliation products by X-ray diffraction was performed on an ARLX’Tra X-ray diffractometer using the following modes and parameters: CuKα radiation with a wavelength of 0.154185 nm; accelerating voltage of 35 kV; X-ray tube heating current of 40 mA. Samples were washed with water and isopane and filtered in wet form and transferred for analysis.

The size of coherent scattering blocks (crystallites, number of layers) was estimated using the Scherrer formula in the approximation of the Lorentzian line shape on the diffraction pattern:

(1)

β is the full width at half maximum of the peak. To determine it, we use the following formula:

(2)

B is the width at half-height of the sample, b is the width at half-height of the line of the standard material (corundum) at the same angle as the experimental line of the sample. The number of layers N _c is determined as:

(3)

where d is the interplanar distance from Bragg's law:

(4)

The angle θ corresponds to 2θ = 26°, the main graphite peak in the diffraction pattern.

A series of peaks at 2θ≈40-50° is necessary to calculate the lateral (in the plane of the layers) size L _a and find the ratio of the 2H and 3R phases.

La is calculated at 2θ ≈ 43.4° for phase 3R and at 2θ ≈ 44.4° for phase 2H from the formula:

(5)

More details on the methodology can be found in N. G. Savinsky, N. S. Melesov, E. O. Parshin et al "Analyzing Products of the Electrochemical Exfoliation of Graphite via Rutherford Backscattering Spectrometry and X-Ray Diffractometry" // Bulletin of the Russian Academy of Sciences: Physics, 2020, Vol. 84, No. 6, pp. 732-735. Fig. 8 shows the diffraction patterns of the original Graflex foil and the exfoliated sample.

3. Methodology for determining the elemental composition of samples using energy dispersive analysis.

The studies were carried out using a high-resolution scanning electron microscope Zeiss Supra-40 with an X-ray energy-dispersive analyzer INCA Energy. The energy range of the registered photons is 0-30 keV. The minimum energy discreteness is 5 eV. The standard resolution of quantitative maps is 1 μm. However, in special cases it can reach 200 nm or less. In addition, a combination of maps of chemical elements and an electron image of the same surface area in elastically scattered electrons can improve the resolution of microanalysis to 10 nm.

The second device for elemental composition analysis was a Quanta 3D 200i dual-beam system with an Apollo X energy-dispersive silicon drift detector (Ametek Inc.) for micro-X-ray spectral analysis. Elemental composition analysis of samples can be performed in the range from the fourth (beryllium) to the ninety-second (uranium) elements with automatic element recognition.

Elemental analysis was carried out at 5 locations on the sample and averaged. When determining the homogeneity of the element distribution, scanning was performed with the formation of quantitative maps of the distribution of chemical elements.

4. Methodology for determining the defectiveness of samples using Raman spectroscopy.

The structure of exfoliated graphite flakes was studied using the EnSpectr R532 Raman spectrometer. Figure 9 shows a typical Raman spectrum of sample 3. Quantitative assessment of structural defects for a wide range of sp ² hybridized systems, including graphene structures, is of critical importance for understanding both fundamental properties and technical applications, primarily charge carrier mobility. It is generally accepted that the peak in the 1580 cm ^-1 frequency range is associated with high-frequency vibrations of E2g optical phonons of the lattice for graphites and nanographites, called the G (graphite) peak. This peak, called the G peak, is observed in the Raman spectrum of any number of graphene layers at 1580 cm ^-1 and is associated with the stretching plane of the C-C bonds of the carbon skeleton of the molecule.

The G band of the Raman scattering spectra of sp ² carbon atoms is observed as a single or multiplicative peak for all sp ² hybridized carbon structures. Defect zones with sp ³ hybridization are characterized by a multiplicative peak at 1350 cm ^-1 , called D (disordered). Its presence is caused by the “breathing” mode of the six-atom aromatic ring A1g and requires a defect for its activation in the first Brillouin zone near the K or K ₁ point. The Raman spectrum of disordered graphite shows two new sharp peaks near 1350 cm ^-1 peak D and 1620 cm ^-1 peak D'. These intervalley D and intravalley D' defect-induced resonance scattering processes are mainly responsible for the electron decoherence in the optical transitions of sp ² carbon atoms. The 2D (or G') band is also defined by sp ² carbon atoms as a single peak or a multiplicative structure in the range of 2500-2800 cm ^-1 . The sensitivity of the 2D peak to the details of the sp ² structure makes this band a powerful indicator for determining the number of graphene layers in a stack. While the G and D' bands have only one peak for any number of graphene layers, the G' band consists of one, four, six, and the D band of two peaks for monolayer, bilayer, trilayer, and multilayer graphene, respectively.

Since absolute intensity is rarely used in Raman spectroscopy, the normalized intensity I _D /I _G is widely used for quantitative measurements of the degree of disorder. It has been observed that the ratio of the intensities D to G varies inversely with the crystallite size, I(D)/I(G) ∞ 1/L _A . Basically, the amount of disorder in a nanocrystallite is determined by the number of boundaries (one-dimensional defects) relative to the total crystallite area Lsp ² . In graphene with zero point defects, the distance between defects, L _D , is a measure of the amount of disorder.

(6)

(7)

(8)

These equations are 6, 7, and 8, obtained to determine the size between the defects of the crystalline regions of graphene L _D and the defect density nD, or in other words, the distance between the defects of the graphene crystal lattice as a function of the energy of the used exciting radiation in the visible range, where the laser excitation is used in terms of wavelength in nm. For the laser with a wavelength of 532 nm, in our study, the defect density nD can be represented by equation 8. For a honeycomb ideal lattice, the number of carbon atoms is (equation 9),

(9)

If we normalize the number of defects in equation 9 by n _c , we obtain the defect concentration per µm ² in parts per million (ppm). This ratio is denoted in the literature as I _D /I _G or I(D)/I(G), while the corresponding area ratio, i.e. the integral ratio of intensity over frequency, is denoted as A _D /A _G or A(D)/A(G).

Fig. 9 shows the Raman spectrum of sample 10; from left to right are peaks D, G, 2D.

Antioxidant additives used for exfoliation of graphite under the conditions of the present invention.

Fig. 5 shows the chemical structural formulas of the used additional substances-additives. The authors conducted studies of the process of electrochemical exfoliation in an aqueous solution of ammonium sulfate of the required concentration, dissolving the required amount of additives of substances from the antioxidant series, such as: TEMPO, IPON, DMSO and classical reducing agents such as: hydrazine hydrate, sodium borohydride, ascorbic acid. The first two compounds are widely used in chemical processes of radical polymerization as a chain growth regulator in radical polymerization processes, a polymerization inhibitor. Samples of the compound 2,2,6,6-tetramethylpiperidine 1oxyl or TANAN, in the English-language literature TEMPO, a representative of the class of stable radicals were provided by the company "Yaroslavl Research Institute "YARSINTEZ" and were used without any additional preparations "as is". Dimethyl sulfoxide (DMSO) of special purity grade was used without preliminary purification; it is a widely known and industrially available bipolar aprotic solvent with the ability to bind and deactivate radicals (scavengers), in particular, the hydroxyl radical.

Fig. 1 shows three stages of the process of electrochemical exfoliation of graphite under the conditions of the proposed invention. The first is intercalation: penetration of guest ions into the interlayer space under the action of a constant electric field. The second stage is expansion. Under the action of the reducing anodic potential, sulfonyl and carbonyl compounds in the interlayer space of graphite are reduced to gaseous products of decomposition of carbon dioxide, sulfur dioxide, oxygen and others. The pressure created by these products in the interplanar cavities expands the graphite. And finally, at the third stage, the splitting off (exfoliation) of individual graphite layers from the massive body of the graphite sample occurs. The upper part of the diagram shows the chemical reactions occurring at the anode: the upper one is the generation of the hydroxyl radical (simplified through the anion radical stage), the lower one at the cathode is the splitting of the water molecule into protons and atomic singlet oxygen. The center shows a diagram of the transformation cycle of the TEMPO scavenger molecule, a trap for the hydroxyl radical. S. Yang, S. Brüller, et al. “Organic radical-assisted electrochemical exfoliation for the scalable production of high-quality grapheme.” J. Am. Chem. Soc., 2015, v.137, pp. 13927–13932, doi: 10.1021/jacs.5b09000.

Figure 2 shows the reaction mechanism of anodic oxidation in aqueous (NH ₄ ) ₂ SO ₄ studied by Parvez et al. in 2014 Parvez K. et al. Exfoliation of graphite into graphene in aqueous solutions of inorganic salts // Journal of the American Chemical Society, 2014, Vol. 136, No. 16. pp. 6083-6091. Hydroxyl ions (OH ^- ) are initially formed during the electrolysis of water, and this strong nucleophile can attack sp ² carbon atoms at the edges of graphite and grain boundaries, forming two vicinal OH groups of reaction (1). II Bilkis, SMShein Investigation of short lived radicals in aromatic nucleophilic substitution by use of nitroso compounds as scavengers //Tetrahedron, 1975, v 31, pp 961-971

Subsequently, the two OH groups can react with each other to form an oxyirane ring, reaction (2). Alternatively, they can dissociate to form two carbonyl groups by further oxidation, as shown in reaction (3) in Figure 4. This results in depolarization and expansion of the graphite layers at the edges, opening the graphite crystal lattice to intercalation by sulfate ions (SO ₄ ) ^-2 , and possibly also more water molecules. In addition to oxidation of the graphite, other reactions can also occur, including the release of CO ₂ and O ₂ by reactions 4 and 5 in Figure 2.

Examples

The examples presented below confirm, but do not limit, the proposed invention.

Example 1 Electrochemical exfoliation of graphite (basic technique).

The exfoliation process was carried out in a thermostatted 0.8 L glass beaker made of heat-resistant Pyrex glass with an electrolyte volume of 0.5 L, in a two-electrode cell, graphite foil (sometimes rolled into a tube) was used as an anode, and metal foils of platinum or stainless steel were used as cathodes. The surface area of the graphite electrode was 15 cm ² . Commercial graphite foil "Graflex" 0.5 mm thick, made of pre-heat-treated intercalated graphite, having a carbon content of 99.5%, sulfur ≤ 0.12, chlorine ≤ 50 ppm, was used after drying at 100 ° C for 5 hours. To avoid short-circuiting of the electrodes during exfoliation, the interelectrode distance was 2 cm. To prepare working solutions of electrolytes, deionized water was used, in which ammonium sulfate of the special purity grade was dissolved in the required concentration, stirring until completely dissolved on a magnetic stirrer. Additives, especially those poorly soluble in water (ammonium vanadate), were added to the aqueous solution with subsequent ultrasonic dispersion until completely dissolved. If necessary, ice cooling was used during ultrasonic exposure. After preparing the electrolyte solution, the graphite anode and stainless steel cathode are lowered into the electrolyte solution, the distance between the electrodes is 2 cm. (For comparison, sample 1 in Fig. 10) A pulse voltage of 10 V is applied to the electrodes, the duration of the anodic and cathodic pulses is 30 s, the duty cycle is 50%.). The exfoliation process ends with the complete destruction of a part of the graphite electrode immersed in the aqueous electrolyte solution and lasts several tens of minutes. To carry out the process according to the claimed method (samples 2-10, Fig. 10), exfoliation is carried out under potentiostatic conditions at 10 V (graphite anode) without applying a pulsed cyclic bipolar potential. During the exfoliation process, gas bubbles are released on both electrodes, and graphite particles pass into the solution. If it is necessary to classify the nanographite sediment by size, centrifugation is used before filtration at a centrifuge rotor speed of 2000-4000 rpm. The fugate is a suspension of nanosized graphite particles in water. Upon completion of the process, the graphite suspension is filtered under vacuum (water-jet pump) through a 2 μm PTFE filter, abundantly washed with water many times to remove electrolyte residues, and then once with isopropanol. Drying in a vacuum drying cabinet for 24 hours at 60 ° C. All samples were analyzed by scanning electron microscopy, X-ray diffraction, Raman spectroscopy and energy-dispersive analysis.

According to the conditions of exfoliation in the proposed invention, the above experimental method is common to all samples. Examples of electrochemical exfoliation conditions and all characteristics of the samples of the obtained nanographite products are shown in Figure 10.

Figure 3 shows the chronograms of the UV spectra of the reaction mixture of sample 7 in the presence of 0.08% DMSO solution in 0.2 M aqueous ammonium sulfate. The graphs demonstrate a decrease in the intensity of the absorption band of the DMSO molecule during exfoliation, which is characterized as DMSO consumption. It is consumed in the process of oxidation by the hydroxyl radical HO* with the formation of a stable non-radical compound of methanesulfinic acid with subsequent oxidation by oxygen to methanesulfonic acid. This mechanism is proposed in the works of A.M.Koulkes-Pujo et.al. “Methan formation from the reaction of hydroxyl radicals and hydrogen atoms with dimethyl sulfoxide (DMSO)” Febs letters v.129, N 1, p.10; J.E.Repine, J.W.Eaton, M.W.Anders, J.R.Hoidal, and R.B.Fox, “Generation of Hydroxyl Radical by Enzymes, Chemicals, and Human Phagocytes In Vitro”// J. Clin. Invest. The American Society for Clinical Investigation, Inc. Vol. 64 December 1979 pp.1642-1651. Figure 4 shows a diagram of the mechanism of hydroxyl radical inhibition by dimethyl sulfoxide with the conversion of DMSO to methanesulfonic acid.

Fig. 10 shows the results of studies using the above-mentioned methods of samples 1-11 of exfoliation of graphite foil (Graflex) under the conditions proposed in the proposed invention.

A significant increase in the calculated values of the defect number density nD and nD/nC is symbatically similar to the behavior of the experimental ratio of the peak intensities ID/IG of the exfoliated graphite (sample 2 in Fig. 10) compared to the value in the initial foil (sample 1 in Fig. 10), and the calculated values of L _sp2 demonstrate a decrease in the size of sp ² domains in the basal plane of the graphene crystal lattice by more than an order of magnitude and a decrease in the size of defects L _D . Such behavior can be easily explained by the processes of splitting the layers of the graphite crystal lattice from 105 to 30 layers during the electro-exfoliation process. However, when the conditions of the exfoliation procedure were changed (sample 3 in Fig. 10), the behavior of the above values, although did not change, but the rate of change significantly decreased. This is primarily due to the introduction of antioxidants into the reaction mass, which block the oxidation processes.

Thus, carrying out the process without radical oxidation inhibitors, but with reducing agents: ascorbic acid, hydrazine hydrate, sodium borohydride leads to approximately the same result. That is, this does not significantly affect the defect density, in contrast to the fact that the use of TEMPO and DMSO significantly reduces the defect density by 2.5-3 times. Probably, a noticeable decrease in the defect density was due to blocking of the hydroxyl radical as an oxidizer of the basal plane of the graphene crystal lattice by radical absorbers DMSO or TEMPO molecules. D. Chapman, 20. A. P. Reuvers, C. L. Greenstock, The mechanism of chemical radioprotection by dimethyl sulphoxide. Int. J. Radiat. Biol. 24, 533-536 (1973). A. M. Koulkes-Pujo et. al. Methane formation from the reaction of hydroxyl radicals and hydrogen atoms with dimethyl sulfoxide (DMSO) Febs letters v.129, N 1, p. 10.

Hydroxyl radicals (HO*) begin to corrode the graphite electrode at the edges, which is necessary to open the boundaries so that sulfate ions can intercalate. When using DMSO, the degree of damage to the graphene lattice, possibly caused by radical attack, can in principle be very low.

It should be noted that for samples (9-11 in Fig. 10) with a high oxygen content, the method for determining defects is incorrect, and therefore defect indicators are not given.

The analysis of X-ray patterns, in addition to those already shown in Fig. 10, is characterized by the absence of a wide halo in the region of 2θ angles of 9-12°, characteristic of graphene oxide synthesized by the Hammer method. This, among other things, demonstrates the low lattice defectivity of the basal plane of graphene in exfoliated graphite samples.

The task set and solved by the claimed method is the creation of a process of anodic exfoliation of graphite in an electrolytic cell with a metal counter electrode (cathode) in an aqueous electrolyte solution containing ammonium sulfate at a temperature of up to 50°C at a constant potential of 10 V due to the penetration (intercalation) of guest ions from the solution under the action of an electric field into the interlayer space of graphite, with subsequent reduction of ions to gaseous compounds that create a wedging pressure in the graphite cavity and tear nanosized particles of graphite from the massive anode into the solution. The use of a wide range of concentrations of an aqueous solution of ammonium sulfate electrolyte, the inclusion of additives regulating the radical-oxidative processes of exfoliation, allows, with an unchanged hardware design of the electrolyte cell, constancy of the electrical parameters of the exfoliation process, to regulate the degree of oxidation of the exfoliation product of nanographite in a wide range of oxygen content, from practically unoxidized to graphite oxide, while monitoring the low defectiveness of nanographite, to obtain a wide range of products at low costs due to widely available chemical raw materials and relatively high productivity of the electrolyzer.

The technical result is the creation of a method for producing nanographite flakes by electrochemical exfoliation with a wide range of nanographite alloying with oxygen from 1.6 to 29 at.% with a universal mode and hardware design of the process, low defectiveness of nanographite, increased productivity of the electrolyzer, expansion of the range of manufactured products, low cost of products due to cheap widely available raw materials and simplicity of technological design.

A comparative analysis of the set of essential features of the claimed method and the set of essential features of analogues and the prototype indicates that the claimed technical solution meets the criterion of "novelty". At the same time, the distinctive features of the invention formula solve the following functional problems.

1) Feature... the exfoliation process is carried out in stationary (constant) conditions: only in an aqueous solution of ammonium sulfate under conditions of constant potential (potentiostatic mode) at 10 V, with unchanged polarity (graphite is the anode of anodic exfoliation) in the absence of a pulse mode of different duty cycles and a gradient of the potential value during the process at a temperature below 50 °C. These indicators accurately characterize the process, while in the analogue of KR 20170064908 only in the description (not in the formula of the invention) are ranges from 20 V to 100 mV and a current from 5 A to 1 mA given. In the prototype, the potential range is very wide 0.01-200 V with clarification from 1 to 30 V, both when using bipolar voltage in the form of cycles, and with a change in the potential value. Such exfoliation modes as multipolarity, variable overvoltage potential, cyclicity fundamentally change the processes occurring on the graphite electrode, do not allow the use of antioxidant additives that control radical oxidation processes and, as a result, do not allow the control of the oxygen content in graphite during exfoliation.

2) The feature... is carried out with a controlled degree of oxygen doping (oxidation) of nano graphite flakes in the range of doping concentrations of 1.6-29 at.% oxygen. In the analogue of KR 20170064908, the problem of chemical grafting of an organic large molecule-anion onto the graphene surface through a double bond and/or bridges of reactive groups of sulfonium, nitro, phosphonium, carbonium, located in the structure of a large organic molecule is solved. The problem of regulating the degree of oxygen doping, which inevitably occurs in aqueous solutions (and even at a high overvoltage of 20 V), is still reflected in the analogue only in the ESCA analysis of oxygen groups and sulfur, as sulfonyl groups during exfoliation, but a quantitative analysis of these quantities is absent. The prototype includes an example of using the antioxidant TEMPO in the exfoliation process, but the characteristics of the exfoliation product such as the intensity of the spectral lines are not given numerically at all. The effect of the antioxidant on the quality of the product can be judged relatively by the thermogravimetric curve, which has a smaller slope of mass loss compared to the product obtained without TEMPO.

3) The feature... is carried out in 0.01-3.0 M aqueous solution of ammonium sulfate in the presence of 0.08-0.1 wt.% dimethyl sulfoxide for a low degree of oxygen doping of nanographite flakes of 1.6-2.2 at.% oxygen. The exfoliation composition in the present invention based on an aqueous solution of an inorganic salt of ammonium sulfate at a concentration of 3.0 M and 0.08 wt.% DMSO differs from the analogue of KR 20170064908, and despite the presence of DMSO in the composition of the exfoliation solution in the prototype, it performs, along with other organic solvents of the polar aprotic nature of propylene carbonate, ethylene carbonate, dimethyl sulfoxide and N-methylpyrrolidone, rather the functions of increasing the dielectric constant, electrical conductivity, penetration and, most importantly, the solubility of large organic molecules in an aqueous-organic solution. While the DMSO content in quantities of 0.08 wt.% in the proposed invention performs the function of a "scavenger" - a trap for the hydroxyl radical (see description) to prevent oxidation of graphite. An insignificant amount of industrially available inexpensive solvent DMSO instead of the specialized antioxidant product TEMPO, used in the prototype as an example, is also attractive for the economy of the process.

4) Sign... for the average degree of oxygen alloying of nanographite flakes 10-17 at.% oxygen, the process is carried out in a 3.0 M aqueous solution of ammonium sulfate. Without additives, the graphite exfoliation modes are unchanged, in other words, universal, and allow alloying with oxygen groups without changing the technological mode and equipment design.

5) Feature… for a high degree of oxygen doping of nanographite flakes 28 - 29 at.% oxygen the process is carried out in 0.01-3.0 M aqueous solution of ammonium sulfate in the presence of 1.3 wt.% ammonium vanadate. In analogs and in the prototype the use of compounds of transition metals, in particular vanadium, is not indicated at all. The oxygen content of 29 at.% is comparable with the processes of obtaining graphene oxide according to Hammer J. Am. Chem. Soc. 80 , p. 1339-1339 (1958) (see characteristics of sample 11 in Fig. 10 for comparison). The disadvantage of the method of synthesis of graphene oxide by the Hammer method is that graphite oxide is a very expensive substance due to the fact that the process of its production requires a large consumption of oxidizers and becomes explosive with an increase in scale. This increases the cost of the graphite oxide obtained by further reduction to graphene. In addition, graphene obtained by reducing graphite oxide has a defective structure, which worsens its electrophysical characteristics (see Fig. 10, sample 11). The mechanism of the exfoliation reaction in the presence of ammonium vanadate requires more thorough and in-depth study, but it is not unexpected. Thus, the processes of oxidation of organic compounds on catalysts - oxides of transition metals are known: M. Ya. Fioshin, I. A. Avrutskaya "Electrooxidation of organic compounds on anodes from oxides of some transition metals" // Uspekhi khimii v. XLIV 1975 g. Vyp. 11 c 2067-2077., US 509209 published 30.04. 74 "Method of preparation of vanadium catalyst for oxidation of organic compounds". The possibility of obtaining graphene oxide by a two-step method: exfoliation in the presence of vanadium compounds, and the second low-energy oxygen functionalization is a potential method for obtaining nanosized monolayer graphene oxide with a low amount of defects, described in J. Cao, P. He, M. A. Mohammed et. al. Two-Step Electrochemical Intercalation and Oxidation of Graphite for the Mass Production of Graphene Oxide // J. Am. Chem. Soc. 2017, 139, 17446-17456. can potentially be applied as the first step.

6) Sign... the method according to any of signs 2-5 allows to obtain nanosized graphite flakes with a low degree of defectiveness of 26-58 ppm/ ^μm2 and dimensions in plan of 50 μm and more. The conducted analysis of defectiveness quantitatively characterizes nanographite as low-defective, which cannot be compared with the data of the prototype and analogs, due to the complete absence of such data.

It should be noted that the yield of the target product as a percentage of the initial graphite cannot be accurately estimated due to changes in the geometric dimensions and mass of the remaining (unused) foil due to significant intercalation of electrolyte ions and incomplete use of the initial graphite foil, which contacts the source above the surface of the electrolyte. With a rough estimate, the yield for all variants of the process is more than 75 wt.% of the initial graphite foil.

This combination of distinctive features allows us to solve the problem and eliminate the shortcomings of the prototype method and analogues, ensuring higher efficiency of the method for producing nano-sized graphite flakes with a controlled degree of oxidation by the method of electrochemical exfoliation of graphite.

Claims (7)

1. A method for producing nanosized graphite flakes with a controlled degree of oxidation by electrochemical exfoliation of graphite, which consists of carrying out the process in an electrochemical cell containing working electrodes made of graphite pre-treated by heat treatment in air, and cathodes-counter electrodes made of stainless steel, in aqueous solutions of electrolytes that are not protic acids, containing ions H ⁺ , NH ₄ ⁺ SO ₄ ^-2 , OH ^-1 , at a temperature of no more than 90°C with cyclic alternating polarity, a potential changing in magnitude from 0 to 30 V, and constant removal of exfoliated particles. 2. The method according to paragraph 1, characterized in that it is carried out with a controlled degree of oxygen alloying - oxidation of nanographite flakes in the range of alloying concentrations of 1.6-29 at.% oxygen. 3. The method according to item 1, characterized in that the exfoliation process is carried out under stationary constant conditions: only in an aqueous solution of ammonium sulfate under conditions of constant potential - potentiostatic mode at 10 V, with unchanged polarity - anodic exfoliation, the anode is graphite in the absence of a pulse mode, different duty cycle and gradient of the potential value during the process, at a temperature below 50°C. 4. The method according to item 1, or 2, or 3, characterized in that for a low degree of oxygen alloying of nanographite flakes of 1.6-2.2 at.% oxygen, the process is carried out in a 0.01-3.0 M aqueous solution of ammonium sulfate in the presence of 0.08-0.1 mass.% dimethyl sulfoxide. 5. The method according to item 1, or 2, or 3, characterized in that for an average degree of oxygen alloying of nanographite flakes of 10-17 at.% oxygen, the process is carried out in a 3.0 M aqueous solution of ammonium sulfate, without additional ingredients. 6. The method according to item 1, or 2, or 3, characterized in that for a high degree of oxygen alloying of nanographite flakes of 28-29 at.% oxygen, the process is carried out in a 0.01-3.0 M aqueous solution of ammonium sulfate in the presence of 1.3 mass.% ammonium vanadate. 7. The method according to any of paragraphs 2-5, characterized in that when exfoliation is carried out in the modes and conditions of paragraphs 2-5, the final product has a low defectiveness of exfoliated graphite, amounting to 26-58 ppt/ ^μm2 with plan dimensions of 50 μm or more.