The O 1s spectra of Na metal dipped in pure PC show two main features at binding energies 531.3 eV and 532.7 eV in a relative intensity ratio of 1:1 (for 2min., Fig. ) and 1:2 (for 2h, Fig, ). Combined, the O 1s and C 1s signature peaks fit the ones expected from the molecular structure of the PC molecule, three chemically different carbon positions and two chemically different oxygen positions. The observed intensity ratios do not match perfectly because, as discussed in the C1s section, some Na 2 CO 3 and other reaction products between Na and PC have formed on the surface. Generally, the O1s region overlaps partially with the Na Auger region when using 1486.6 eV as excitation energy in XPS. Therefore, all O1s spectra include two characteristic Auger peaks for sodium at 524 eV and 536 eV. When moving from the Na metal submerged in pure PC to Na metal in electrolytes based on NaClO 4 in PC without (Figure ) and with VC (Figure ) the shape of the O1s spectra changes significantly indicating more pronounced reactions between the metal surface and the electrolytes than between the metal surface and the pure solvent. The prominent O1s peaks around 531 eV could be fitted with three components. For both samples submerged for only two minutes, a clear peak at low binding energies is observed. This peak at 530.0 eV is attributed to NaOH. The main peak centered at 531.1 eV is assigned to a carbonate species such as contained in Na 2 CO 3 and alkyl carbonates. Because the intensity of this peak in the O1s for carbonate is too high in relation to the -CO 3 peak in the C 1s, this indicates that there is likely another species contributing to this peak. As observed in the C 1s this could indicate alkoxide species like CH 3 ONa, leading to an O1s peak at 531.7 eV binding energy. After considering the alkoxide species contribution to the peak intensity at 531.1 eV, the relative amount of oxygen from the CO 3 peak is in good agreement with the C 1s peak for Na 2 CO 3 .