A comparative study is made on thermal stability of two solid electrolyte interphase (SEI) films on a graphite negative electrode. The SET films are generated by reductive decomposition of two different electrolyte solutions that are prepared by dissolving either lithium hexafluorophosphate (LiPF6) or lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) in ethylene carbonate/diethyl carbonate solvent. The surface film derived from the LiTFSI-containing electrolyte, which is enriched by organic carbon-oxygen species with uniform coverage, remains rather intact in both chemical composition and film morphology after high-temperature (85 degrees C) storage, indicative of a superior thermal stability. As a result, this SET layer maintains its passivating ability even after the storage. In contrast, the film generated from the LiPF6-containing electrolyte is thermally degraded with a substantial change in both film morphology and chemical composition to lose its passivating ability upon high-temperature exposure. The poorer thermal stability of the latter film has been ascribed to a high population of Li- and F-containing inorganic species and presence of cracks or void spaces in the SET layer. (C) 2013 The Electrochemical Society. All rights reserved.