Temperature dependence of the passivation layer on graphite

The elevated temperature stability of the Solid Electrolyte Interface (SEI) formed on graphite during the first charge/discharge cycle has been investigated. This was done in order to determine its role in the high-temperature degradation process which occurs in a C/LiMn2O4 Li-ion cell. X-ray photoelectron spectroscopy (XPS) is used to probe the surface-layer growth and elemental composition of graphite electrodes exposed to different thermal treatment. The surfaces of cycled electrodes, when stored below 60 degrees C, were seen to resemble closely those of unstored electrodes. An electrode stored at 60 degrees C exhibited a significant increase in the amount of oxidized carbon and oxygen. Analysis by Ar sputtering suggests that a thick 'macroscopic' layer coats the graphite electrode surface, but not the separate graphite grains; this is consistent with the observed rapid decrease in capacity and large cell-resistance on cycling such cells after storage. This capacity decrease was not observed for the cells stored at RT and 40 degrees C. (C) 1999 Elsevier Science S.A. All rights reserved.