Ellipsometer Studies of Surface Layers on Lithium

Abstract

The growth of surface layers on lithium in propylene carbonate solutions can be followed by ellipsometry, although the refractive indexes of many potential film materials are close to those of the electrolyte. Film thicknesses calculated from ellipsometer measurements increase over periods of several days at open circuit; they are several times larger than those derived from galvanostatic pulse measurements. Films are found to be inhomogeneous with properties continuously varying as a function of distance from the substrate; compact regions are located adjacent to the metal and porous regions are located adjacent to the solution. Electrode capacitance measurements are sensitive to the thin compact region, which can also be generated by reaction with water vapor. Ellipsometer measurements are primarily affected by the thicker, porous region, which may be formed by the precipitation of decomposition products of the solution.