2025 AIChE Annual Meeting

(536h) The Influence of Temperature on Solid Electrolyte Interphase Formation and Ammonia Selectivity in Lithium-Mediated Nitrogen Reduction

Authors

Peter Benedek, Stanford University
Thomas Jaramillo, Stanford University
Over the past decade, the lithium-mediated nitrogen reduction reaction (Li-N2R) has emerged as a promising approach for ammonia (NH3) synthesis using renewable electrical energy, enabling on-site production under near-ambient conditions. The reaction mechanism begins with the electrodeposition of lithium (Li) from an organic electrolyte, followed by nitrogen (N2) activation on metallic Li and subsequent reaction with added protons (H+), resulting in NH3 formation. A critical aspect of this system is the formation of a solid-electrolyte interphase (SEI), which regulates the transport of Li+, H+ and N2 to the electrode surface and the release of NH3 back into the electrolyte. Therefore, the chemical composition, selective transport properties, thickness, and mechanical characteristics of the SEI are pivotal in determining long-term stability, NH3 selectivity, and overall yield.

In this study, we investigate the influence of temperature on the surface and bulk species composing the SEI, using two Li salts dissolved in diglyme with 1 vol% ethanol acting as the proton source. Sustained current measurements at varying temperatures revealed that for the lithium tetrafluoroborate (LiBF4)-based electrolyte, the SEI composition changed significantly with temperature, though faradaic selectivity towards ammonia (FENH3) remained constant at approximately 12% up to 50°C. In contrast, with the lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)-based electrolyte, the FENH3 increased gradually with temperature, reaching a maximum of 14% at 80°C.

To further characterize the SEI, we perform a three-fold SEI rinsate analysis using nuclear magnetic resonance (NMR), inductively coupled plasma mass spectrometry (ICP-MS) and ion chromatography (IC), quantifying organic and inorganic SEI species and estimating the SEI thickness after Li-N2R. Additionally, air-free X-ray photoelectron spectroscopy (XPS) measurements provided insight into parasitic reactions, identifying side products that may explain differences in SEI thickness, as well as variations in NH3 selectivity and yield. Furthermore, we observe fluctuations in faradaic efficiency (FE) at elevated temperatures, with FENH3 reaching up to 40% over short periods of time. These fluctuations correspond to different stages of SEI formation, degradation and reformation. Introducing current dosing treatments reduced these oscillations, likely by diminishing side reactions while improving FENH3 under increased temperatures. These findings offer a deeper understanding of the material composition and dynamic behavior of the SEI in response to temperature during Li-N2R advancing our understanding of its role in controlling NH3 selectivity and production.