The solid-electrolyte interphase (SEI) is often invoked to rationalize trends observed during lithium-mediated electrochemical ammonia synthesis (LiMEAS), but quantitative analysis of the SEI constituents is lacking. In this work, we quantified lithium alkoxide SEI species (derived from proton donor reduction) under a variety of electrolyte compositions, using ethanol as the standard proton donor along with five alternative linear and branched alcohol proton donors. With ethanol, we showed that Faradaic efficiency linearly increased from ∼10 to ∼20% as the lithium ethoxide layer thickened. When comparing different proton donors, we found that FE exhibited a “volcano plot” dependence with lithium alkoxide SEI layer thickness, such that FE increased with increasing SEI thickness up to ∼4 μm and decreased with increasing SEI thickness thereafter. We concluded the lithium alkoxide SEI layer limited proton donor and solvated Li+ transport, resulting in an optimal SEI thickness at which proton donor, solvated Li+, and N2 transport were appropriately tuned to maximize NH3 selectivity.