In situ spectroscopy reveals how water-driven SEI formation controls selectivity in Li-mediated N2 reduction

Abstract

Understanding the fundamental processes that govern the formation of the solid electrolyte interphase (SEI) layer in lithium mediated nitrogen reduction is crucial to the design of improved electrolyte formulations. In this work, the roles of water and ethanol on the formation of the SEI are studied using in situ infrared spectroscopy and postmortem cross-sectional microscopy. Our results connect the observed SEI morphologies to specific observed SEI formation mechanisms. We directly detect formation of lithium ethoxide (LiEtO) as a major SEI component at potentials positive of Li plating when ethanol is used as the proton donor, which further reacts to form LiOH and Li₂O depending on water availability, and regenerates ethanol. We show that in water-containing electrolytes, the SEI consists of a highly porous outer layer, which we propose is largely LiOH, and a denser inner layer, consisting largely of Li₂O. We show that the water concentration relative to the ethanol concentration in the electrolyte can strongly influence the porosity of the SEI, which in turn influences N₂ reduction selectivity. Furthermore, our combined approach of directly probing SEI formation in real time and measuring morphological changes to the SEI can provide a framework for more informed SEI engineering to unlock further optimisation of the Li-mediated system.