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-depend ent morphological changes at a highly oriented pyrolytic graphite (HOPG)/el ectrolyte interface. Cyclic voltammetry was performed on an HOPG crystal co vered 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 measure ments 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 produ cts concentrate at the HOPG edge planes. X-ray photoelectron spectroscopy c haracterization 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 s alt (here, LiBF4), organic solvent and water are discussed. (C) 2000 The El ectrochemical Society. S0013-4651(00)03-097-4. All rights reserved.