A Comparative Study on Thermal Stability of Two Solid Electrolyte Interphase (SEI) Films on Graphite Negative Electrode

A comparative study is made on thermal stability of two solid electrolyte interphase (SEI) films on a graphite negative electrode.The SEI films are generated by reductive decomposition of two different electrolyte solutions that are prepared by dissolving either lithium hexafluorophosphate (LiPF 6 ) 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 C) storage, indicative of a superior thermal stability.As a result, this SEI layer maintains its passivating ability even after the storage.In contrast, the film generated from the LiPF 6 -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 SEI layer.