THE LOW TEMPERATURE OXIDATION OF LITHIUM THIN FILMS ON GRAPHITE BY O2, H2O, AND CO

Abstract

Lithiated graphite and lithium thin films are used in fusion devices. In fusion devices, lithiated graphite will undergo oxidation by background gases. In order to gain insight into this oxidation process, thin (< 3 nm) lithium films on highly oriented pyrolytic graphite (HOPG) were exposed to O2(g), H2O(g), and CO(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), H2O(g), and CO(g) exposure. Auger electron spectroscopy (AES) was used to obtain the relative initial sticking coefficients during O2(g), H2O(g), and CO(g) exposure.

AES experiments showed that as the lithium film thickness decreased from 15 to 5 to 1 ML, the initial sticking coefficients decreased for O2(g), H2O(g), and CO(g). AES measurements showed that a 15 ML lithium film was fully oxidized after 9.7 L of O2(g) exposure and Li2O was formed. Additionally, HREEL spectra showed that, even at 100 K and low exposures (< 0.5 L), O2(g) fully dissociated upon reacting with the lithium thin films. AES measurements showed that the 15 ML lithium film did not fully oxidize after 15 L of H2O(g) exposure, but the 5 and 1 ML lithium films did fully oxidize. HREELS showed that during initial exposure (< 0.5 L) of H2O(g), lithium hydride and lithium hydroxide was formed on the surface of a 15 ML lithium film. After 0.5 L of H2O(g) exposure, the H2O(g) began to physisorb. AES measurements also showed that a 15 ML lithium film was fully oxidized by CO(g) after 11.0 L of CO(g) exposure. Additionally, HREELS measurements have shown no evidence that CO(g) formed either a physisorbed or a chemisorbed Li—CO complex, but showed evidence of Li—O species present in the surface. This suggests that the CO(g) fully dissociated upon reacting with the lithium thin films on HOPG. Finally, O2(g), H2O(g), and CO(g) oxygen uptake experiments by 15 ML Li films on HOPG showed that H2O(g) had the highest relative initial sticking coefficient and CO(g) had the lowest relative initial sticking coefficient.