Operando and multiscale insights into the solid electrolyte interphase in lithium-mediated nitrogen reduction.

Lithium-mediated nitrogen reduction reactions (Li-NRRs) have emerged as a promising route for ambient ammonia electrosynthesis, yet their performance is fundamentally governed by the solid electrolyte interphase (SEI). Far from being a simple passivation layer, the SEI is a dynamic region that regulates Li+ transport, N2 access, proton delivery and parasitic reactions, thereby controlling activity, selectivity and stability. Its precise role remains difficult to establish as the SEI is thin, heterogeneous, metastable and continuously evolving during operation. In this Review, we examine the origins and functions of the SEI and discuss an integrated operando, in situ and multiscale characterization framework to resolve its role in Li-NRR. We highlight advances in techniques including X-ray scattering, Raman and infrared spectroscopies, neutron reflectometry, cryogenic electron microscopy, time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and titration methods, which provide complementary insight into SEI composition, structure and evolution. Finally, we discuss the limitations of SEI characterization, SEI analysis integrated with product detection, a practical workflow for SEI studies in Li-NRR, and future directions towards understanding SEI formation and evolution, with the aim of enabling its rational design for efficient and durable lithium-mediated ammonia synthesis.