In Situ Investigations of SEI Layer Growth on Electrode Materials for Lithium-Ion Batteries Using Spectroscopic Ellipsometry

Thin film electrodes of a-Si and TiN were prepared by magnetron sputtering for use with a custom-designed tubular in-situ spectroscopic ellipsometry (SE) electrochemical cell. Ellipsometric measurements on Li/a-Si in-situ cells with 0.1 M LiPF6/EC:DEC (1:2) electrolyte were made while cells were charged and discharged. Large reversible changes in the ellipsometric parameters, Ψ and Δ, were closely related to the state of charge of the in-situ cell. The transformation of semiconducting a-Si to metallic a-LixSi alloy and the formation of an SEI layer contributed to these large changes, which we were unable to decouple. Li/TiN cells were made so that the SEI forming process could be studied without competing reactions. The effect of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) additives on SEI formation were studied. SEI thicknesses for cells containing no additives, VC, and FEC were roughly 18 nm, 25 nm and 30 nm, respectively, after a 10 h hold at 0.1 V vs Li/Li+.