Optical Study of the Surface Film Formed during Li-Metal Deposition and Dissolution Investigated by Surface Plasmon Resonance Spectroscopy.

The features of the electrode surface film during Li-metal deposition and dissolution cycles are essential for understanding the mechanism of the negative electrode reaction in Li-metal battery cells. The physical and chemical property changes of the interface during the initial stages of the reaction should be investigated under operando conditions. In this study, we focused on the changes in the optical properties of the electrode surface film of the negative electrode of a Li-metal battery. Cu-based electrochemical surface plasmon resonance spectroscopy (EC-SPR) was applied because of its high sensitivity to optical phenomena on the electrode surface and its stability against Li-metal deposition. The feature of SPR reflectance dip depends on the optical properties of the electrode surface; namely, the wavelength and depth of the reflectance dip directly connected the refractive index and extinction coefficient (color of electrode surface film), which was confirmed by reflectance simulation. In the operando EC-SPR experiment, various changes in optical properties were clearly observed during the cycles. In particular, the change in the extinction coefficient was more remarkable at the second process than the first process of Li-metal deposition. By electrochemical quartz-crystal microbalance (EQCM) measurements, surface film formation was confirmed during the first Li-metal deposition process. The remarkable change in the extinction coefficient is based on the color change of the surface film, which is caused by the chemical condition change during Li-metal deposition cycles.