Thermal stability of the HOPG/liquid electrolyte interphase studied by in situ electrochemical atomic force microscopy

In situ atomic force microscopy (AFM) was used to follow temperature-dependent morphological changes at a highly oriented pyrolytic graphite (HOPG)/electrolyte interface. Cyclic voltammetry was performed on an HOPG crystal covered with an electrolyte [1 M LiBF4, ethylene carbonate/beta-butyrolactone (EC/gamma-BL) 2:1] with a 0.5% water content Water reduction was observed at 1.4 V vs. Li/Li+ A solid electrolyte interphase (SEI) formation occurred at 0.8 V,and lithium-ion intercalation began at 0.2 V. In situ AFM measurements were made at 25, 40, 50. 60, and 70 degrees C. Destruction or melting of the SEI layer, causing a growth of small blisters spread on the surface, could be detected at 50 degrees C. At higher temperatures, reaction products concentrate at the HOPG edge planes. X-ray photoelectron spectroscopy characterization of HOPG cells stored at 20 and 80 degrees C support die in situ AFM observations. Graphite powder electrodes show similar thermal SEI breakdown behavior, generating a thick carbon-oxygen layer on the electrode surface at temperatures above 60 degrees C. The influence of the lithium salt (here, LiBF4), organic solvent and water are discussed. (C) 2000 The Electrochemical Society. S0013-4651(00)03-097-4. All rights reserved.