Reactivity of lithium toward nonaqueous solvents of relevance to energy storage applications as studied by surface analytical techniques

The interactions between metallic lithium and perdeuterated tetrahydrofuran (TDF) have been examined in ultrahigh vacuum (UHV) by X-ray photoelectron and Auger electron spectroscopies (XPS and AES, respectively) and temperature-programmed desorption (TPD). The AES, XPS, and TPD spectra of thick layers of Li vapor deposited on clean polycrystalline silver (Ag(poly)) foils exposed to up to 100 langmuirs of TDF at room temperature showed no evidence for features other than those associated with clean Li/Ag(poly) surfaces. This indicates that the sticking coefficient of (gaseous) TDF on Li at ca. 300 K is negligibly small. Clearly defined C(1s) and O(1s) XPS features were observed, however, for submonolayer coverages of TDF (θ(TDF) < 1) on Li/Ag(poly), obtained by first condensing a thick layer of TDF below 140 K and later gently warming the specimen to ca. 300 K. The binding energies of these peaks were smaller than those observed for condensed TDF recorded at ca. 135 K, for which the features were identical to those found for θ(TDF) < 1 on bare Ag(poly) at the same temperature. The m/e = 48 TPD spectra of TDF condensed on Ag(poly) yielded for θ(TDF) < 1 a well-defined peak centered in the range 210-240 K, a temperature much higher than that associated with the desorption of bulklike TDF (ca. 170 K). Very similar results were obtained for experiments involving Li/Ag(poly) surfaces, except that the m/e = 48 peak was broader and slightly shifted to higher temperatures. The fragmentation patterns of adsorbed TDF (either on Ag(poly) or Li/Ag(poly)) were consistent with cyclopropane and formaldehyde as the most likely thermally desorbed species. Unlike the behavior observed for TDF/Ag(poly) for which no TPD features were detected for T > 300 K, the m/e = 4 TPD spectra for TDF/Li/Ag(poly) displayed a prominent peak centered at about 600 K attributed to D₂ generated by the thermal decomposition of LiD. This provides rather unambigous evidence that (condensed) TDF reacts with Li at temperatures lower than ca. 350 K to yield LiD as the main product.