XPS Study of the Interface Evolution of Carbonaceous Electrodes for Li-O2Batteries during the 1st Cycle

The surface chemistry of the commonly employed positive electrode substrate Super C65 carbon was investigated during the 1st cycle of a Li-O2 battery with a typical ether electrolyte (0.2 M LiTFSI in Diglyme) by performing in situ online electrochemical mass spectrometry (OEMS), ex situ scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS). During discharge, a nanometer-thin ( 5 nm) layer of Li2O2 forms homogeneously throughout the electrode before it is passivated at a final specific charge of ∼365 mAh/g. Higher discharge charge rates lead to thinner and more densely packed layers of Li2O2. Electrolyte decomposition also occurs in parallel, as evidenced by the continuous increase of LiF and sulfur containing species of the degraded LiTFSI salt. On charge, O2 evolves initially according to a 2e−/O2 rate, but as the cell over-potentials increase, O2 evolution rate approaches 4e-/O2, thus demonstrating a significant extent of parasitic side-reactions. Unlike the discharge, the charge leads to inhomogeneous electrode reactions causing a continuous removal of Li2O2 with no indication of LiO2 or Li2O intermediates. The decomposition side-products formed during discharge are also removed and the spectra of the pristine electrode are nearly retained at a full charge. Our results show that the analysis of Li-O2 battery chemistry requires broad approach based on the combination of various electrode surface sensitive techniques, such as XPS and OEMS, in order to provide further fundamental understanding of the mechanisms behind the complex electrochemistry involving the Li and O2 as well as several other predominant degradation products of the electrode and electrolyte.