Surface reactivity of graphite materials and their surface passivation during the first electrochemical lithium insertion

The surface passivation of TIMREX (R) SLX50 graphite powder was studied as received and after heat treatment at 2500 degrees C in an inert gas atmosphere by differential electrochemical mass spectrometry in electrochemical lithium half-cells. I M LiPF6 in ethylene carbonate and either a dimethyl carbonate, propylene carbonate or 1-fluoro ethylene carbonate co-solvent was used as electrolyte systems in these half-cells. The SEI-film formation properties of both graphite materials were correlated with their active surface area (ASA), being responsible for the interactions between the carbon and the electrolyte system. The active surface area was determined from the amount of CO and CO2 gas desorbed at temperatures up to 950 degrees C from the graphite material surface after chemisorption of oxygen at 300 degrees C. The structural ordering at the graphite surface increased significantly during the heat treatment of the SLX50 graphite material as indicated by the significant decrease of the ASA value. The increased surface crystallinity was confirmed by krypton gas adsorption, Raman spectroscopy as well as temperature-programmed desorption. This increased structural ordering seemed to be the parameter being responsible for a hindered passivation of the heat-treated SLX50 causing partial exfoliation of the graphite structure during the first electrochemical lithium insertion in the ethylene carbonate/dimethyl carbonate electrolyte. In the case of the ethylene carbonate/1-fluoro ethylene carbonate electrolyte system, primarily the fluoro compound is responsible for the graphite passivation. In this electrolyte system, pristine SLX50 and the less reactive, heat-treated SLX50 graphite showed significantly different SEI-film formation mechanisms. In contrast, no difference in the passivation mechanism could be identified for different graphite surfaces in the ethylene carbonate electrolyte system with propylene carbonate as co-solvent. (c) 2005 Elsevier B.V. All rights reserved.