Panteleimonov R. Electrochemical properties of graphene-graphite-metaloxide systems

The dissertation is devoted to: the synthesis of graphene-containing structures from aqueous and non-aqueous (liquid nitrogen) media by the plasma- arc discharge method and the study of the influence of the synthesis medium on the change in the physical and chemical properties of graphene-containing materials; development of experimental samples of cathodes for lithium-ion batteries using graphene-graphite-metal oxide systems. In this dissertation, the influence of synthesis conditions on the structural and morphological features of graphene-containing materials, changes in surface morphology, the chemical and electronic state of atoms on the surface of graphene- containing materials, their phase, component composition, and thermal stability. To study the characteristics of graphene-like structures, as well as their influence on the capacitive and conductive characteristics of electrodes for lithium- ion current sources, physicochemical methods and electrochemical testing were applied, which combine the possibility of simultaneous measurement of electrical and kinetic parameters of all systems. When studying the structural-morphological and thermal properties of graphene and graphene-graphite-metal oxide systems, it was established that a change in the synthesis environment leads to a change in the structural- morphological properties of graphene. A number of physical and chemical studies of graphene-containing materials obtained from aqueous and liquid nitrogen mediums have proven that the dimensions of nitrogen-containing graphene (G-N) are about 50-100 nm with a specific surface area of 23 m 2 /g with a number of layers of about 20, while graphene from an aqueous medium (G-H 2 O) has twice the size of about 200-300 nm and a specific surface area of 27.7 m 2 /g with a number of layers of about 40. The X-ray photoelectron spectroscopy method characterized new nitrogen doping centers on the surface of graphene nanostructures, such as: a typical graphene structure (sp 2 C–sp 2 C), two atypical structures (sp 3 C–N and a C–O bond) and typical nitrogen-modified graphene components (pyridine–N, pyrrole–N, graphite–N and oxidized N–O). The degree of structural perfection of graphene based on the ratio of D-mode intensity to G-mode intensity is 0,126 and 0,06, depending on the synthesis environment. A decrease in the value of this ratio indicates an increase in the degree of perfection of nanostructures. The analysis of the experimental values of the specific electrostatic capacity and active resistance of the samples showed that the presence of water in graphene affects the average values of the capacity relative to graphene without water, depending on the mass ratio and heat treatment. Heat treatment at 250 o C of graphene synthesized from an aqueous medium lead to an increase in capacitance and conductivity values. In pure samples of graphene and graphite, the capacity is lower than in the combination of these materials in a mixture. An increase in the content of graphene leads to an increase in electrostatic capacity. The presence of water in graphene is the main factor affecting the resistivity of graphite Gr/graphene G-H 2 O systems, and the capacitance value depends on the amount of graphite in the mixture. Measurements of the electrical conductivity of graphite Gr - graphene G-N mixtures in different mass ratios and under the influence of temperature treatment established the general preservation of higher values of specific conductivity with an increase in the proportion of graphene. An electrochemical study of prototype samples of chemical current sources based on graphite-graphene - spinel compositions established the following factors that regulate the electrochemical performance of cathode materials based on them: graphene G-H 2 O impurities increase the share of electronic conductivity, and graphene G-N and spinel impurities increase the ionic conductivity relative to graphite ; the type of graphene regulates the capacitive indicators of current sources by reducing the activation energy of the redox reaction involving lithium ion. It is shown that the capacitive characteristics obtained with spinel XDM exceed other characteristics of materials by 20%. And with carbonate spinel, the capacitive characteristics are 30% higher than in materials with graphene synthesized from a nitrogen environment. A comparative analysis of graphene- spinel systems with graphite-spinel systems was carried out, which showed a complete loss of capacity of systems with graphite at high current loads over 6C.