Surface Characterization on Lithium Insertion∕Deinsertion Process for Sputter-Deposited AgSn Thin-Film Electrodes by XPS

Abstract

In this paper, the structural and surface changes of the sputter-deposited thin-film electrodes during Li insertion∕deinsertion have been investigated by exsitu X-ray diffraction and X-ray photoelectron spectroscopy (XPS). The study revealed a possible type of capacity decay mechanism for the Sn-based alloy electrodes. It was considered that the surface reactions of active material with electrolyte led to the formation of solid electrolyte interphase (SEI) layer in the first Li insertion process, which was damaged during the Li deinsertion process due to the strong shrinkage of the inner active material. Thus, in the Li deinsertion process, the electrolyte could readily permeate through the damaged SEI layer and reacted with the inner active material to form tin oxides. These tin oxides were conductively isolated, leading to the consuming of active material and capacity decay. It was found that the degree of the formation of tin oxides was strongly related to the identity of electrolyte. Much more SEI layer and tin oxides were formed in the propylene carbonate (PC)-based electrolyte than in the ethylene carbonate (EC)-based electrolyte, due probably to their different reactivity with the active material. This could be the reason why both the sputter-deposited thin film and the powder-pasted alloy electrodes showed rather poor cycling stability in the PC-based electrolyte.