Although there are differences due to different kinds of anode materials used between lithium and sodium-based batteries (graphite versus hard carbon), the SEI is a crucial component for the functionality and safety of alkali batteries and has been studied extensively using experimental and theoretical methods . It has been demonstrated that the electrolyte composition significantly impacts SEI formation. Numerous studies have shown the importance of the solvent mixture 8 9 10 , as well as the used electrolyte salt 11 12 . Further, the beneficial effects of additives like FEC or VC on the stability of the SEI have been established 13 14 15 16 . Therefore, all electrolyte components contribute to the formation of the SEI, the salt, the charge carrier, the solvent, and the additives 17 18 . However, the mechanisms through which the different components influence the growth of the SEI still need to be fully understood. Resolving said mechanisms has been the focus of many studies, ranging from atomistic simulations to experimental post-mortem analysis 19 20 21 22 . The complexity of the SEI poses a challenge for both, experimentalists and theoreticians alike. Combining the different sizes and timescales often complicates any direct comparison between the atomistic view of theoretical simulations, which typically operate on the picosecond timescale, and the macroscopic scale of experiments, which frequently operate on timescales spanning days or weeks. Here, we attempt to unravel one piece of the puzzle: the influence of sodium perchlorate on the SEI formation in sodium (metal) batteries using propylene carbonate as the electrolyte solvent. We combine atomistic scale density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations with X-ray photoelectron spectroscopy (XPS) and gas chromatography (GC) measurements to develop a detailed understanding of the initial SEI formation steps in this system.