The low temperature oxidation of lithium thin films on HOPG by O2 and H2O

Abstract

Lithiated graphite and lithium thin films have been used in fusion devices. In this environment, lithiated graphite will undergo oxidation by background gases. In order to gain insight into this oxidation process, thin (<15monolayer (ML)) lithium films on highly ordered pyrolytic graphite (HOPG) were exposed to O2(g) and H2O(g) in an ultra-high vacuum chamber. High resolution electron energy loss spectroscopy (HREELS) was used to identify the surface species formed during O2(g) and H2O(g) exposure. Auger electron spectroscopy (AES) was used to obtain the relative oxidation rates during O2(g) and H2O(g) exposure. AES showed that as the lithium film thickness decreased from 15 to 5 to 1ML, the oxidation rate decreased for both O2(g) and H2O(g). HREELS showed that a 15ML lithium film was fully oxidized after 9.7L (L) of O2(g) exposure and Li2O was formed. HREELS also showed that during initial exposure (<0.5L) H2O(g), lithium hydride and lithium hydroxide were formed on the surface of a 15ML lithium film. After 0.5L of H2O(g) exposure, the H2O(g) began to physisorb, and after 15L of H2O(g) exposure, the 15ML lithium film was not fully oxidized.