Lithium-mediated nitrogen reduction reaction (Li-NRR) has emerged as a promising alternative to the conventional Haber–Bosch process, enabling modular and decentralized ammonia production. A critical component influencing Li-NRR efficiency is the solid-electrolyte interface (SEI), which modulates reactant transport and controls reaction pathways at the electrode surface. Recently, tetraalkyl organic salts have attracted attention as advantageous proton carriers due to their superior electrochemical stability and structural versatility, offering avenues for optimized SEI control. In this study, we utilized in situ attenuated total reflectance–surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) to dynamically investigate SEI formation during Li-NRR, comparing ethanol (EtOH) and tetrabutylammonium chloride (TBACl) as proton carriers. Distinct differences in SEI composition and morphology were observed, with EtOH promoting rapid and extensive formation of irregular organic SEI layers, predominantly lithium ethoxide (LiOEt) and lithium carbonate (Li2CO3). Conversely, TBACl conditions suppressed organic SEI growth through stable electric double layer formation by TBA+ ions, significantly limiting free THF molecules near the electrode surface and, thus, reducing unwanted organic SEI components. Our results highlight the critical role of proton carrier selection in controlling SEI formation, emphasizing TBACl’s advantages for enhancing Li-NRR stability and efficiency.