Modulation of Sodium-Metal Interphase through Ionic Liquid Electrolyte Chemistry and Formation Current

This study investigates the impact of cation type and formation current density on the properties of the solid electrolyte interphase (SEI) for sodium-metal electrodes utilizing two ionic liquid electrolytes. Here, N-isopropyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide ([Ci3mpyr][FSI]) and diethyltetramethylguanidinium bis(fluorosulfonyl)imide ([DETMG][FSI]) with 30 mol % NaFSI are studied for the first time as sodium electrolytes by means of electrochemical analysis and the characterization of their SEI composition. Four interphase formation protocols with varying current densities were tested in Na||Na symmetrical cells, and the interfacial behavior is discussed. The SEI formed in each system was characterized in situ, using electrochemical impedance spectroscopy (EIS) and plating–stripping cycling, and ex situ, with scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDX) and X-ray photoelectron spectroscopy (XPS). Despite the [Ci3mpyr][FSI]-based electrolyte exhibiting superior bulk ionic conductivity and higher Na+ mobility, the [DETMG][FSI] electrolyte outperformed it in terms of lower cell polarization, enhanced cycling stability, and the absence of short-circuit failures. Among the tested protocols, a moderate formation current density of 0.5 mA/cm2 was found to be optimal for both electrolytes, yielding a balanced SEI with the highest concentration of inorganic species and compact morphology. This study underscores the critical role of IL cations’ structure in SEI formation on Na metal at varying currents, providing insights into future design of cations and formation protocol optimization.