Improve Stability and Selectivity with Sulfur-Derived SEI for Li-Mediated Nitrogen Reduction for Green Ammonia Production

Ammonia is a major compound widely used for commercial, and agricultural purposes and has recently been considered as the hydrogen carrier. Specifically, the annual consumption of ammonia reaches 200 million tons, demonstrating its significance. However, ammonia has been produced from the Haber-Bosch process, high temperature (400-500 °C), and pressure conditions (up to 200 bar), leading to many environmental (~1.4% of global CO₂ emissions) and energy-related (∼1% of global energy use) issues.
Lithium-mediated nitrogen reduction (Li-NRR) is considered the most feasible method for obtaining production quantities distinct from impurities. It utilizes the low electronegativity of Li to break the strong triple bond of nitrogen. The mechanism of Li-NRR has three steps: 1) lithium plating, where Li metal is deposited; 2) lithium nitridation, where Li reacts with nitrogen and forms lithium nitride (Li₃N); and 3) protonation, where lithium nitride reacts with a proton donor (such as ethanol) and finally produces ammonia. For this cyclic mechanism to perform robustly, the properties of the Solid Electrolyte Interphase (SEI), which is formed by decomposition of the electrolyte during lithium plating (the first step) at the electrode interface, must be controlled during reaction. It could determine not only diffusivity of reactants like nitrogen but also uniformity of lithium plating, finally selectivity of ammonia production in Li-NRR. Furthermore, chemical stability and mechanical strength of the SEIs are related with the electron-consuming decomposition of SEI during lithium plating involving stability of the whole reaction. Thus, electron insulating, ion-permeable and mechanically robust SEI is needed to promote Li-NRR.
In this study, we investigate sulfur-derived SEI in Li-NRR for the first time. Dimethyl sulfide (DMS) was chosen as the sulfur source due to its simple chemical structure. DFT calculation also shows DMS has lower LUMO energy levels than other components of the electrolyte. Thus, it reduced well to form sulfur-derived SEI. The addition of a small amount of DMS to the electrolyte altered the physicochemical structure of the SEI with Li₂SO4 and Li₂S resulting in improved lithium-ion conductivity and mechanical strength. The sulfur-derived SEI led to a more uniform and large lithium deposition and changed the morphology of the SEI from a vertically oriented film-like structure to a net-like structure. The study demonstrated that the sulfur-added electrolyte maintained a more stable cell potential during longer times (20 hours) than the base electrolyte (10 hours). This indicates that the introduction of sulfur enhances the overall stability in Li-NRR, enhancing energy efficiency. Similarly, the electrolyte decomposition decreases in the sulfur-added electrolyte. In addition, the sulfur-added electrolyte achieves higher ammonia production and Faradaic efficiency (390 ± 50 μg cm^–2 h^–1, 46 ± 6%) than the base electrolyte (305 ± 34 μg cm^–2 h^–1, 36 ± 4%). Modifying the SEI through sulfur addition shows promise in improving the Li-NRR process.