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°C, were seen to resemble closely those of unstored electrodes. An electrode stored at 60°C exhibited a significant increase in the amount of oxidized carbon and oxygen. Analysis by Ar sputtering suggests that a thick ‘macroscopicʹ 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°C.