Multimodal Operando Interrogation of Sodium-Ion Interphases: Uncovering Cation-Dependent Solid Electrolyte Interphase Dynamics

Abstract Ionic liquid (IL) electrolytes offer significant promise for sodium-based batteries due to their safety and electrochemical stability. This study applies a multimodal in situ experimental method to investigate the evolution of the interface on a model electrode in order to investigate how variations in IL cation chemistry influence electrolyte solvation structure, electrode–electrolyte interfacial processes, and solid electrolyte interphase (SEI) formation and stability. Using in situ Raman spectroscopy, electrochemical quartz crystal microbalance (EQCM), outer-sphere redox probe assays, and differential capacitance measurements, imidazolium, phosphonium, and pyrrolidinium-based IL electrolytes with identical sodium bis(fluorosulfonyl)imide (NaFSI) salt, mixed in a low concentration of 10 mol %, have been compared. The results demonstrate that cation identity critically controls ion coordination environments, electrical double layer, SEI growth dynamics, and stability, which in turn govern electrochemical behavior observed in cyclic voltammetry. Notably, at these salt concentrations, phosphonium-based electrolytes promote faster and immediate SEI formation, imidazolium-based systems develop thinner and unstable interphases, while pyrrolidinium-based electrolytes form a steady, stable, and inorganic-rich SEI layer characterized by rapid equilibrium and tentative FSI–-reduction-derived inorganic signatures. These findings provide mechanistic insights into electrolyte–electrode interactions and offer pathways for characterizing these as a function of electrolyte composition thereby facilitating the future design of high-performance electrolytes for advanced batteries.