Sirosh V. Salt-solvate electrolytes for high-capacity lithium batteries with sulfur-containing cathode materials

The dissertation is devoted to the study of physico-chemical and electrochemical properties of salt-solvate solutions and their application as promising electrolytes for lithium power sources with conversion sulfur-containing cathode materials. The subject of research was to determine the specific characteristics of iron disulfide and elemental sulfur and their stability during cycling in cells with a lithium anode using electrolyte solutions of different composition and concentration, as well as the effect of experimental temperature. A relatively new class of electrolytes based on salt-solvates, which has an acceptable level of specific conductivity, wide range potentials of electrochemical stability, high thermal stability and chemical resistance, was chosen as the electrolyte system. The choice of salt-solvate solutions is due to their low solvating ability in relation to redox-active particles of the positive electrode and at the same time the ability to ensure high mobility of lithium ions (Li+). The use of such electrolytes for electrochemical systems such as lithium-iron disulfide (Li||FeS2) and lithium-sulfur (Li||S) contributes to a significant reduction in the solubility of lithium polysulfides (Li2Sn), which are intermediate products of reduction-oxidation of the active cathode material, due to lack of free solvent in their composition, and variation in the anionic and concentration composition of the lithium salt in salt-solvate solutions has a significant effect on the mechanism of electrochemical reactions of Li2Sn. It was established that the magnitude of the specific electrical conductivity of salt-solvate solutions at low temperatures is almost independent of the nature of the anion of lithium salt. As the temperature rises, this dependence is significantly increased in the LiTf < LiBF4 < LiClO4 < LiTFSI series, which is similar to increasing the degree of dissociation of lithium salts. The results of the high thermal stability of such solutions provide for the possibility of significant expansion of the temperature range of operation of lithium batteries. An attempt has been made to systematically approach the choice of lithium salt for electrolyte systems, which can be effectively applied in lithium batteries with a conversion sulfur-containing cathode. The electrochemical characteristics of the sulfur electrode in salt-solvate solutions based on tetraglyme in the conditions of room temperature and elevated temperatures have been determined. The optimal concentration of the salt component in salt-solvate electrolytes is established, which is at 0.4 m.f., in which the lithium-sulfur galvanic pair shows the highest results. A new composite cathode for Li||S system based on carbon monofluoride has been developed. It is shown that the introduction into the composition of the positive electrode is about 20% by weight CFx additive can significantly increase the specific characteristics due to the possible improvement of the conductivity in the structure of the cathode and the adsorption of polysulfides on the surface of amorphous carbon. The investigated salt-solvate solutions can be used for practical application in rechargeable lithium batteries with a conversion cathode based on the iron disulfide or elemental sulfur, operating at relatively low current densities. Keywords: salt-solvate, electrical conductivity, conversion cathode, lithium battery, carbon monofluoride, adsorption, polysulfides.