Evidence for infrared absorption enhancement of species at the lithium metal/electrolytic solution interface

Infrared ATR spectroscopic measurements have been made to investigate electrochemical species at the interface between metallic lithium and propylene carbonate (PC) solution containing 0.5M LiClO4. The metal was electrodeposited on the internal reflection plane of a Ge prism at −0.2 V (versus Li/Li+. Lithium deposition was demonstrated by measuring both the cyclic voltammogram and light reflectivity simultaneously. The angle of incidence (θ) was varied from 25° to 70° in the ATR measurements. Remarkably intensified bands were observed at 1260 and 1020 cm−1 in p- and s-polarized ATR spectra measured at 0.35 V. Before Li deposition, the former band was extremely weak and the latter was not observed. Irrespective of polarization, the 1020 cm−1 band exhibited a maximum intensity at θ = 40° at which the background reflectivity was minimum, whereas the intensity of the 1260 cm−1 band increased with decreasing θ and reached a maximum at 30°. Most part of the enhancement observed for these bands is explained in terms of the transverse collective resonance of free electrons in the Li film.