Due to its important role in the production of fertilizers ammonia is one of the most produced chemicals worldwide. In recent years there has been increasing interest in the electrochemical lithium mediated nitrogen reduction reaction (LMNRR), as a possible alternative to the Haber-Bosch process. The LMNRR is advantageous as it can be carried out under ambient temperatures and pressures [1]. The LMNRR process involves electrochemical plating of metallic Li from an organic electrolyte. This plated Li reacts with elemental nitrogen forming an intermediate nitride species, which is subsequently protonated to form ammonia. To ensure high selectivity towards ammonia production the competing hydrogen evolution reaction has to be limited [2]. The solid electrolyte interphase (SEI), formed from electrolyte decomposition at the working electrode plays a crucial role in this as it influences mass transport to the electrode [3]. Studies using neutron reflectometry with one-minute time resolution investigated the initial stages of SEI formation in LiClO₄ containing electrolytes [4]. Recent investigations show improved performance using LiBF₄ containing electrolytes [5]. In this study formation and dynamics of the SEI layer derived from LiClO₄ and LiBF₄ containing electrolytes were examined in operando using grazing incidence wide-angle X-ray scattering (GI WAXS) in a synchrotron setup. The use of GI WAXS provides a time resolution in the seconds range, enabling the identification of transient species and fluctuations of deposits on the electrode surface. Different SEI species and their dynamics have been observed and correlated with electrolyte composition and faradaic efficiency towards ammonia. Figure 1a) shows the dynamic behavior of diffraction features attributed to Li and LiF in an experiment using LiBF₄ as electrolyte salt. In Figure 1b) the development of the corresponding peak intensities is shown and correlated with the start and end of chronopotentiometry (CP), used to study the cell. LiF was formed from the decomposition of LiBF₄ and accumulated on the working electrode, showing that LiF is a significant component of the SEI in fluoride-containing electrolytes. Furthermore, metallic Li was found to be more stable when LiBF₄ was utilized as electrolyte salt, in contrast to LiClO₄. This finding suggests that the SEI derived from LiF has different mass transport characteristics and is more robust than that derived from LiClO₄. This may explain the previously reported superior performance of electrolytes containing LiBF₄ [5]. References [1] S. Z. Andersen et al., “A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements,” Nature, vol. 570, no. 7762, pp. 504–508, Jun. 2019, doi: 10.1038/s41586-019-1260-x. [2] S. Z. Andersen et al., “Increasing stability, efficiency, and fundamental understanding of lithium-mediated electrochemical nitrogen reduction,” Energy Environ. Sci., vol. 13, no. 11, pp. 4291–4300, Nov. 2020, doi: 10.1039/d0ee02246b. [3] N. Lazouski, K. J. Steinberg, M. L. Gala, D. Krishnamurthy, V. Viswanathan, and K. Manthiram, “Proton Donors Induce a Differential Transport Effect for Selectivity toward Ammonia in Lithium-Mediated Nitrogen Reduction,” ACS Catal., pp. 5197–5208, Apr. 2022, doi: 10.1021/acscatal.2c00389. [4] S. J. Blair et al., “Lithium-Mediated Electrochemical Nitrogen Reduction: Tracking Electrode–Electrolyte Interfaces via Time-Resolved Neutron Reflectometry,” ACS Energy Lett., vol. 7, no. 6, pp. 1939–1946, Jun. 2022, doi: 10.1021/acsenergylett.1c02833. [5] K. Li et al., “Increasing Current Density of Li-Mediated Ammonia Synthesis with High Surface Area Copper Electrodes,” ACS Energy Lett., vol. 7, no. 1, pp. 36–41, Jan. 2022, doi: 10.1021/acsenergylett.1c02104. Figure 1