Surface Chemical and Electrochemical Processes of Well-Characterized Lithium Thin Films: Electrochemical Reactions in SO₂/CH₃CN

The electrochemical discharge of lithium thin films, prepared in ultrahigh vacuum, has been explored in SO₂-saturated CH₃CN solutions. Prior surface characterization of the products of the reactions between clean lithium and O₂, H₂O, SO₂, and various solvents, using x-ray photoelectron spectroscopy, are reviewed, as they pertain to the kind of surface chemical reactions which occur to the clean lithium surface, after its formation, and prior to its immersion into the electrolyte. The interaction of the clean surface with the backfill gases, and the gases above the solution leads to a predominately sulfite surface layer, overlaying a thin oxide/sulfide product layer. The electrochemical discharge characteristics of this surface, compared with a thin film which had seen 10²–10⁴ Langmuirs of O₂ exposure, showed that the initial oxide formed in the vacuum chamber had a significant effect on both the steady-state potential achieved by the anode, as well as the time necessary to achieve that steady-state potential. Interestingly, oxide formed from pure O₂ exposures had a more passivating effect on the lithium thin film, with respect to electrochemical discharge, than did comparable pre-exposure to H₂O. The effect of these oxide layers is also compared with the effect of films formed on the lithium surface as a result of exposure to the electrolyte over a period of a few hours.