Correlation Between Li Solvation and the Interfacial Chemistry for Enhancing Lithium-Mediated Electrochemical Ammonia Synthesis

Ammonia (NH3) is a cornerstone chemical in modern society, primarily used as fertilizers (80%) and as the source of N in all synthetic chemicals. Ammonia production, being the largest in the chemical industry, exceeds 170 million metric tons annually, relies on the 100-year-old Haber Bosch process which operates at elevated temperatures (450 °C) and pressures (100 bar)[1]. The harsh operation conditions not only make it among the “Big Four” industries that release half of the industrial emissions in the world, but also demands a large-centralized infrastructure whose prohibitive capital cost renders ammonia inaccessible in many developing countries. Electrochemistry provides a key advantage as it allows to drive chemical transformations at milder conditions and using renewable feedstocks. As the only reliable system to date[2-3], the Lithium mediated electrochemical N2 reduction to ammonia (LiNRR) in a non-aqueous electrolyte has emerged as a promising candidate. In LiNRR, the activation of molecular nitrogen (N2) is preceded by the electrochemical deposition of metallic lithium in an organic solvent, which then reacts with N2 and undergoes protonation to release NH3. Progress in recent years aimed at improving the faradaic efficiency, energy efficiency and long-term stability have been achieved via changing the salt [4-5] and proton donor compositions, which have been correlated with the structure and composition alterations of the Solid Electrolyte Interphase (SEI). However, the understanding of which lithium salts and electrolyte compositions are optimal for LiNRR is unclear and scattered across different groups, owing to the different experimental conditions. More importantly, fundamental understanding of the interplay between N2 reduction, parasitic reactions and speciation at the interface that drive efficiency and system instability is lacking. In this study, we aim to draw a comprehensive connection between lithium solvation structures, its implications on SEI formation, and the corresponding ammonia faradaic efficiency and electrode stability of LiNRR. A variety of lithium salts and compositions in Tetrahydrofuran and Ethanol were employed to evaluate their efficacy for ammonia synthesis under the same transport conditions. Physical descriptors to probe Li+ solvation structure were quantified via electrochemical Li/Li+ redox characterization and vibrational spectroscopy of the bulk electrolytes. Our study revealed a clear correlation between the extent of Li+-anion & Li+-solvent pairing, NH3 faradaic efficiency and the degree of parasitic reactions. We also bridged the connection between bulk electrolytes and LiNRR performance through the post-mortem electrode characterization via Raman and Nuclear Magnetic Resonance (NMR) spectroscopy. Such techniques shone light into the distinct SEI chemistry engendered by solvent-rich vs anion-rich Li solvation as well as the different degrees of ethanol participation in SEI formation. An order-of-magnitude improvement in faradaic efficiency can be obtained at the optimal electrolyte composition compared to the state-of-the-art ClO4 or BF4 electrolytes. By drawing a comprehensive connection between ammonia efficiency against bulk and interfacial descriptors, we could reveal the fundamental origin of different NH3 synthesis performances in different electrolytes which will aid in engineering stable interfaces, resulting in a long-lasting, high-performing system for practical applications. [1] Smith, C., Hill, A. K. & Torrente-Murciano, L. Energy Environ Sci 13, 331–344 (2020). [2] Iriawan, H., Andersen, …, Chorkendorff, I., & Shao-Horn, Y. Nature Reviews Methods Primers, 1(1), 56. (2021). [3] Cai, X., Fu, C., Iriawan, H., …, Shao-Horn, Y., & Zhang, J. IScience, 24(10), 103105. (2021). [4] Li, S., Zhou, Y., Li,…, Nørskov, J. K., Chorkendorff, I. Joule, 6(9) (2022). [5] H. L. Du, M. Chatti, R…, D. R. MacFarlane, A. N. Simonov, Nature 609, 722 (2022).