Simultaneous microgravimetric and optical reflectivity studies of lithium underpotential deposition on Au(111) from propylene carbonate electrolytes

The underpotential deposition (upd) of lithium on highly oriented Au(111) electrodes was examined in propylene carbonate (PC)-LiAsF6 solutions by simultaneous in situ quartz crystal microgravimetry (QCM), UV-visible reflectance spectroscopy (ΔR/R), and cyclic voltammetry (CV). An analysis of the QCM and ΔR/R data was performed in the potential range 0.4 < E < 2.5 V vs Li /Li⁺(PC) after the voltammetric response had reached steady state, i.e., following the irreversible formation of a film on the electrode surface. The results obtained indicated that both the (background corrected) charge under the Li upd stripping peak and the loss in the effective mass of the QCM (Δm_eff) are consistent with the oxidation of one to two monolayers equivalent of Li on a smooth Au(111) substrate. The optical reflectivity recorded during these experiments revealed large, on the order a few tenths of a percent, and wavelength dependent changes in ΔR/R as a function of the applied potential. A combination of these data with those derived from the coulometric/microgravimetric analysis of the Li upd stripping peak yielded, for Li coverages of about 0.1, values of Δ R/R as large as 0.3% at wavelengths of about 650 nm. A comparison between the experimental ΔR/R spectra at fixed Li coverages, and that predicted theoretically based on an ideal, non-interacting, three-layer model, indicates that the features observed are mostly due to 1 /R contributions due to the bare Au substrate.