AN IN-SITU INVESTIGATION OF SOLID ELECTROLYTE INTERPHASE FORMATION ON ELECTRODE MATERIALS FOR LITHIUM-ION BATTERIES USING SPECTROSCOPIC ELLIPSOMETRY

Abstract

A novel method to detect and quantify the growth of the solid electrolyte interphase (SEI) on battery electrode materials using in-situ spectroscopic ellipsometry (SE) is presented. The effects of additives in 1 M LiPF6/EC:DEC (1:2) electrolyte on the SEI were studied. Thin film electrodes of a-Si, Ni, and TiN were prepared by magnetron sputtering for use with a custom-designed tubular in-situ electrochemical cell. Li/a-Si and Li/Ni in-situ cells in 0.1 M LiPF6/EC:DEC (1:2) were studied by galvanostatic chronopotentiometry. Large changes in the ellipsometric parameters, ? and ?, were observed for both materials. These changes were closely related to the state of charge of the in-situ cell. The formation of an a-LixSi alloy, the formation of an SEI layer, or both contributed to these large changes for a Li/a-Si in-situ cell. For a Li/Ni in-situ cell, a thin transparent surface layer was observed. The surface layer, presumably made from SEI species and species from the displacement reaction between NiO and Li, increased to roughly 17 nm during the first discharge. During the first charge, the surface layer thickness decreased to roughly 5.5 nm and could not be removed, even at high potentials. The effect of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) additives on SEI formation were studied using a Li/TiN in-situ cell in 1 M LiPF6/EC:DEC (1:2) by potentiostatic chronoamperometry. 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. SE is a useful technique for measuring thin film growth in-situ on electrode materials for Li-ion batteries.