This work showcased a combined theoretical and experimental approach toward understanding the initial degradation steps leading to forming the solid electrolyte interphase in sodium batteries. We were able to link the different scales, both in terms of size and time, between atomistic scale modeling and gas chromatography by correlating the adsorption behavior of decomposition products with their detectability within the GC setup. We further showed that AIMD simulations spanning just a few ps, depicting perchlorate decomposition on alkali metal surfaces and initial stages of phase separation, could be extrapolated and confirmed by X-ray photoelectron spectroscopy. By combining the insights gained from experimental measurements and theoretical simulations, we could not only identify changes in the decomposition process of the electrolyte but also propose a possible mechanism by which these changes occur. Specifically, we explain the sudden formation of propylene oxide upon adding sodium perchlorate into an electrolyte containing propylene carbonate in contact with a sodium metal surface. This formation was found to be linked to NaCl appearing on the sodium metal, which in turn enables a ring-closing reaction for the readily formed precursor of PO. The precursor was found to be driven by a carbonate-forming reaction rather than an oxide-forming reaction. The abundance of sodium carbonate and sodium chloride within the XPS measurements supports these findings. This study provides an initial step to understand electrolyte decomposition, it provides detailed mechanistic insights that may be applicable beyond the scope of this electrolyte-salt combination. Further work on the effects on performance and lifetime is needed, though. The formation of inorganic compounds, such as fluorides or carbonates, within the SEI of alkali metal batteries and their direct impact on the stability of the SEI through properties like solubility or elasticity is well established. However, secondary effects of such evolving structures, like the sudden formation of epoxides, as shown in this study, still need to be further understood. The appearance of additional electrolyte decomposition products should not be underestimated since molecules like epoxides can enable polymerization reactions and lead to significant changes in the structure and properties of the SEI.