Evaluating electrochemical impedance spectroscopy for SEI characterization in lithium- and sodium-metal batteries

Electrochemical impedance spectroscopy (EIS) is widely used to probe the solid electrolyte interphase (SEI) in alkali-metal batteries, yet the interpretation of impedance responses remains challenging due to the simultaneous influence of interfacial transport properties and evolving electrode morphology. Here, we systematically evaluate the capability of EIS to characterise SEI behaviour in symmetric lithium- and sodium-metal cells using carbonate and ether electrolytes. Time-resolved impedance measurements were combined with distribution-of-relaxation-times analysis, equivalent-circuit modelling, temperature-dependent EIS, and in situ NMR spectroscopy to track interfacial evolution during formation, cycling, and rest. EIS captures key interfacial changes, including resistance growth during open-circuit conditions and capacitance increases associated with expansion of the electrochemically active surface area during cycling. However, comparison with in situ NMR reveals that continuous microstructure formation and SEI accumulation can occur without being directly reflected in impedance-derived SEI parameters. Temperature-dependent analysis further shows that activation energies derived from resistance may include geometric contributions associated with surface-area changes, whereas analysis of the interfacial time constant provides a more surface-area-independent metric of SEI transport. These results demonstrate that while EIS is a powerful tool for monitoring interfacial evolution, impedance responses in metal-electrode systems must be interpreted carefully and supported by complementary techniques to distinguish intrinsic SEI properties from morphology-driven effects.