Lithium deposition on polycrystalline silver. A comparison between electrochemical and gas-phase environments

The electrochemical properties of clean and oxygen-contaminated polycrystalline Ag surfaces have been examined in LiClO₄/polyethylene oxide solutions in ultrahigh vacuum (UHV) environments at temperatures in the range 323-333 K. Unlike the behavior observed for Au and Ni under the same experimental conditions, no clearly defined voltammetric peaks were found during the first and subsequent cycles in the range 2.20-0.25 V vs. Li/Li⁺ initiated at the open-circuit potential, 1.75 V vs. Li/Li⁺. Instead, the scans in the negative direction were characterized by two adjoining regions in which the current increased linearly with potential, albeit at different rates, and the subsequent scans in the positive direction yielded comparatively much smaller currents largely independent of the applied potential. Integration of the voltammetric curves over the potential range 0.25<E<2.20 V vs. Li/Li⁺ revealed a pronounced imbalance between the charges obtained in the scans in the negative (Q_-) and positive (Q⁺) directions. This phenomenon was attributed, by and large, to the high rates of Li dissolution into Ag at these temperatures, consistent with the presence of a low-temperature eutectic in the Li-Ag phase diagram. Additional support for this view was obtained from UHV non-electrochemical measurements involving vapor-deposited Li onto Ag, for which the amount of Li on the surface, as monitored by Auger electron spectroscopy, decreased markedly upon increasing the temperature from ca. 300 to 350 K. The voltammetry of oxygen-contaminated Ag surfaces was characterized by a well-defined peak in the scan in the positive direction centered at ca. 1.3 V, which persisted upon continuous cycling. Although the process responsible for this feature has not yet been identified, it provides a marker for detecting oxygen impurities on Ag in this electrolyte.