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XPS Study of the Porous Carbonaceous Positive Electrode Interface in Li-O2 Batteries upon Cycling
0.2 M LiTFSI in diglyme was chosen as electrolyte for our study. In order to evaluate the influence of the nature of the carbon cathode on the electrochemical properties of the cell, electrodes containing different carbons (graphite SFG6 (TIMCAL), amorphous carbons Super C65 (TIMCAL)) were prepared by mechanical rolling using PTFE as a binder. The specific charge at the end of the 1st discharge was lower for graphite SFG6 (76-96 mAh/g) than for Super C65 (~ 350 mAh/g). It is consistent with the lower specific surface area of the graphite SFG6 (16 m2/g) compared to Super C65 (60 m2/g).
XPS data were collected on the carbonaceous electrodes at the end of the first discharge. For all electrodes the component belonging to the carbon backbone dominates the C 1s spectra (Figure 1c), indicating a thin interface layer (≤5 nm). LiF was detected at the surface of both electrodes but in a higher amount for graphite SFG6 (Figure 1a). Components at 54.8 and 531.7 eV on the Li 1s and O 1s spectra for the electrode containing amorphous carbon Super C65 can be attributed to the presence of Li2O2 (Figure 1a and 1b), but were not observed for graphite SFG6. Hence the type of carbon has an influence on the chemical composition of the interface layer formed on its surface upon discharge. In both cases, electrolyte degradation could be evidenced by presence of components on the C 1s spectrum at 287, 289 and 290 eV resulting from degradation of the diglyme. Upon the following charge, most of the discharge products disappear.