Electrified interfaces are critical to the performance of energy systems and often demonstrate substantial complexity under operating conditions. A nanoscale understanding of the interfacial microenvironment, i.e., the solid-electrolyte interphase (SEI), in lithium-mediated nitrogen reduction (Li–N2R) is key for realizing efficient ammonia (NH3) production. Herein, we used time-resolved neutron reflectometry (NR) to observe SEI formation under Li–N2R conditions. We found that the LiBF4-based electrolyte provided a substantially more well-defined SEI layer than previous SEI NR interrogations that used LiClO4, highlighting the underlying chemistry that dictates electrolyte design and enabling new NR-based studies. Using in situ NR, we found that the LiBF4-derived SEI under Li–N2R conditions comprises a thick, diffuse outer layer and a thin, compact inner layer at low current cycling (