Addressing the challenges of Li-ion and post-Li-ion batteries, including Na-ion variants, requires improvements in both electrolyte and electrode materials. This necessitates detailed studies of the system behavior in both the bulk liquid and solid phases, as well as the solidelectrolyte interphase (SEI). In Na-ion batteries, the solid phase typically features hard carbon as the anode material, while the cathode is commonly made from Prussian blue/white and their analogues, layered transition metal oxides, or vanadium phosphate . Regarding the liquid phase, carbonate-based organic electrolytes have been proposed, often with small amounts of additives that help stabilize the SEI on anode and cathode during cycling . However, these electrolytes share similar safety concerns with Li-ion batteries due to the presence of highly flammable organic carbonates . Non-flammable ionic liquids address this safety risk while also offering a sufficiently wide electrochemical stability window, excellent thermal stability, and low vapor pressure . However, they are typically constrained by high production costs and relatively low ionic conductivity . As an alternative, water-in-salt (WiS) electrolytes have recently gained attention . Although these electrolytes may provide lower electrochemical stability compared to ionic liquids, they benefit from reduced production costs by using water, the most abundant solvent in nature, and excellent transport properties that often surpass those of ionic liquids . Additionally, their nonflammable nature ensures high safety, similar to ionic liquids, making them promising candidates for advanced energy storage systems with enhanced safety requirements. While many experimental and computational studies have elucidated the main electrochemical processes occurring in bulk electrodes and bulk electrolytes , the complex reactive behavior at the interface remains unclear. In particular, the formation mechanism and structural properties of the SEI, a crucial component that significantly impacts electrochemical stability and charge/discharge processes , are not yet fully understood. Although early studies have provided insights into SEI quality through performance measurements , they lack detailed molecular-scale information on the SEI characteristics. Subsequent research utilized various ex situ techniques, such as vibrational and photoelectron spectroscopy and microscopy methods, alongside computational approaches like molecular dynamics (MD) simulations, to further investigate SEI structure in various electrolytes, with a particular focus on ionic liquids . While these studies offer insights into the SEI composition, they lack information on the SEI formation process, particularly in its initial stages. To address this gap, Forster-Tonigold et al. combined their experimental measurements with density functional theory (DFT) calculations to investigate the interactions between the 1-butyl-1-methylpyrrolidinium bis(trifluoromethane-sulfonyl)imide (BMP-TFSI) ionic liquid and bulk-like sodium and magnesium films. This approach provided an atomic-scale perspective on the reactive interactions between the examined ionic liquid and the anode models, elucidating the initial stage of SEI formation. Similarly, Stottmeister and Groß employed ab initio molecular dynamics simulations based on DFT calculations to study the early stages of SEI formation during the decomposition of typical solvent molecules on alkaline metal anodes. Building on these insights, further research is needed to investigate SEI composition and formation processes across various electrolyte/electrode systems, including promising variants of WiS electrolytes, to optimize the performance of Na-ion batteries.