ИСТИНА

Among different kinds of lithium batteries, commercially available or being under investigation, lithium-oxygen system has received increased attention in the last few years due to its potential ability to demonstrate few-fold enhanced energy performance in comparison to lithium-ion ones. The question of its practical applicability, however, remains open due to a number of challenges have to be addressed, including chemical instability of a positive electrode during cell operation. Carbon materials, which are commonly used as cathode in Li-O2 cells because of their free availability, porosity, high conductivity and light weight, were found to be unstable in presence of superoxide intermediate species produced in oxygen reduction reaction during battery discharge thus limiting sustainable cycling of the positive electrode. Recently non-oxide conducting binary compounds of titanium, including TiC and TiN, attracted researchers’ attention as alternative cathode materials due to their commercial availability in the form of nanopowders, high conductivity and reasonable price. According to a recent report, TiC powder cathode demonstrated high cycle stability with Li2O2 as main discharge product, which was attributed to a thin TiO2 layer growing during cell operation and protecting electrode surface from further oxidation by O22- and O2- species. Further investigations, however, revealed strong dependence of recharge performance of both TiC and TiN to TiO2 surface layer thickness with optimal value estimated to be about 2 nm. It is high enough to inhibit the oxidation but still low to maintain reasonable electron transport. In this work we present a detailed analysis of surface chemistry in TiC – Li – O system by photoemission spectroscopy. We show that clean TiC surface is oxidized by O2 and Li2O2 but can be easily protected from progressive reactions by a natural oxide layer that in addition to oxides and oxycarbides contains significant amount of elemental carbon. Nevertheless, we also demonstrate that such layer is not practically useful as carbon is oxidized in the presence of Li-O2 battery discharge intermediates. We anticipate that dense uniform carbon-free TiO2 layer of 2-3 nm thickness on TiC surface will be a promising solution, and thus further efforts should be taken for developing synthetic routes enabling preparation of TiC/TiO2 core-shell structures.