Consequently, it is unlikely that the whole reaction occurs on any one surface. However, a possible reaction path is found if one considers the non-static nature of the forming SEI. Assuming that reaction step 1 occurs on newly exposed sodium metal or a sodium-rich surface, the intermediate propanolates should readily form. During the decomposition of the perchlorate-containing electrolyte, the initial sodium metal surfaces transform into increasingly chloride-rich surface structures, eventually forming sodium chloride. During the chlorination of the sodium surfaces, the adsorbed propanolate is destabilized, resulting in the ring-closing reaction leading to the formation of propylene oxide. Figure depicts the pro- This ongoing SEI formation reaction and Na-metal exposure to the electrolyte is supported by the consistently increasing pressure during long-term storage experiments, demonstrating constant CO formation . It is important to note that within this work, we focus on the thermodynamics of the proposed reaction pathway. One would also have to consider the kinetics to understand the reaction completely. A proper study including nudged elastic band (NEB) calculations was deemed beyond the scope of this work, however. In the following section, we employed XPS measurements to investigate the interphase between sodium metal and our electrolyte mixtures to confirm or disprove the presence of NaCl in the interphase.