Lithium-mediated electrochemical ammonia synthesis (LiMEAS) is a way-to-go to reduce nitrogen at ambient pressure and temperature, thus avoiding harsh conditions of industrial standard Haber-Bosch process.¹ Inspired by Tsuneto,² we showed how the choice of electrolyte solvent has a considerable effect both in terms of ammonia yield and electrolyte degradation, which affects the long term stability.³ Recently, we introduced a strategy to improve LiMEAS by pulsed instead of constant current over time leading to considerably higher Faradaic efficiency.⁴ The pulsed current kept the electrode cleaner from deposit, which allowed the system to run for a longer time. The thinner deposit decreased the ohmic losses, thus increasing the energy efficiency of the electrosynthesis process. Including our discovery that small amounts of oxygen (O₂) added to nitrogen (N₂) enhance Faradaic efficiency of LiMEAS,⁵ the role of the solid-electrolyte interphase (SEI) has to be understood, in order to make LiMEAS stable, reliable, and more efficient. The SEI is responsible for performance and stability of Li-ion batteries (LIB).⁶ It grows on the working electrode (WE) once the deposited lithium contacts the electrolyte resulting in the materials from electrolyte decomposition. Inorganic, organic and polymeric compounds are the main components of this layer, which can be controlled. The same is expected in LiMEAS, since the organic byproducts in electrolyte are formed during electrosynthesis.³ In an ideal case, only Li⁺ and H⁺ should diffuse through this layer, shielding the reactive electrode against further reduction reactions.⁷ However, the composition, behavior and specific role of SEI in LiMEAS is not known, and the field lacks of studies on it. In this work we show what SEI in LiMEAS is and how its formation and properties can be tuned. An operando electrochemical quartz crystal microbalance (EQCM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), gas chromatography-mass spectrometry (GCMS) and nuclear magnetic resonance (NMR) were employed to determine dynamic, mechanical and chemical SEI properties as a function of time, electrolyte, gases, and additives. We focus on the physical behavior and chemical composition changes on the electrodes and in the electrolyte. The role of SEI was elaborated in detail by comparing electrochemical operation conditions, constant and pulsed current, introducing additives to the electrolyte or exchanging electrolyte components. When the current is pulsed, electrolyte decomposition triggers slower and more controllable interphase growth with some mass stripping during relaxation compared to rapid growth in constant current LiMEAS. Due to these (electro-)chemical processes, the interphase formed on cathodes with pulsed current is thinner, less rigid and more viscous than formed with constant current. Furthermore, addition of O₂ to N₂ suppresses the electrolyte degradation and chemical side reactions, leading to a more efficient and chemically stable LiMEAS. High performance of LiMEAS with optimal O₂ was attributed to the formation of a stable and efficient SEI on the surface of the working electrode, and the subsequent suppression of HER from electrolyte. The main phases in SEI were lithium hydroxides (LiOH∙xH₂O), carbonate (LiCO₃), and ethoxide (LiOEt). However, larger masses in EQCM and NMR describe polytetrahydrofuran (polyTHF) network formation. This work additionally elaborates on the effect of LiMEAS and consequent SEI on the electrolyte decomposition. Our results point towards efficient, stable and robust LiMEAS with highly dynamic structure–activity–stability relationship of the electrode interphase for, but not limited to, LiMEAS. C. Li, T. Wang and J. Gong, Transactions of Tianjin University, 2020, 26, 67-91. A. Tsuneto, A. Kudo and T. Sakata, Chem. Lett., 1993, 22, 851-854. R. Sažinas, S. Z. Andersen, K. Li, M. Saccoccio, K. K., J. B. Pedersen, J. Kibsgaard, P. K. K. Vesborg, D. Chakraborty and I. Chorkendorff, Submitted, 2021. S. Z. Andersen, M. J. Statt, V. J. Bukas, S. G. Shapel, J. B. Pedersen, K. Krempl, M. Saccoccio, D. Chakraborty, J. Kibsgaard, P. C. K. Vesborg, J. Nørskov and I. Chorkendorff, Energy & Environ. Sci., 2020, 13, 4291-4300. K. Li, S. Z. Andersen, M. J. Statt, M. Saccoccio, V. J. Bukas, K. Krempl, R. Sažinas, J. B. Pedersen, V. Shadravan, Y. Zhou, D. Chakraborty, J. Kibsgaard, P. C. K. Vesborg, J. K. Nørskov and I. Chorkendorff, Science (New York, N.Y.), 2021, 374, 1593-1597. S. K. Heiskanen, J. Kim and B. L. Lucht, Joule, 2019, 3, 2322–2333. L. Wang, A. Menakath, F. Han, Y. Wang, P. Y. Zavalij, K. J. Gaskell, O. Borodin, D. Iuga, S. P. Brown, C. Wang, K. Xu and B. W. Eichhorn, Nat. Chem., 2019, 11, 789-796.