Chemistry and Formation of Solid-Electrolyte Interphase (SEI) at Li-Metal Potential

Li metal batteries (LMBs) have long been considered a solution to address the energy density limitations of Li-ion batteries. However, the widespread adoption of LMBs has faced challenges due to operational issues, notably safety concerns arising from the formation of Li dendrites and uncontrolled reactivity at the interfaces, leading to continuous consumption of the electrolyte. To enhance the reversibility of the Li-metal anode (LMA), a promising approach has emerged involving the development of a novel liquid electrolyte comprising an ether solvent and LiFSI salt. Notably, this electrolyte formulation has achieved impressive Coulombic efficiency (CE) values surpassing 99.5%. Despite this progress, the underlying causes of Coulombic inefficiency (CI) and its connection with the formation of a solid-electrolyte interphase (SEI) on the Li metal surface remain inadequately understood. Gaining fundamental insights into these essential processes occurring during continuous cycling of the LMA can provide valuable guidance for optimizing CE more effectively. This presentation focuses on recent advancements that systematically unravel the underlying chemistry and formation of SEI derived from FSI anions at the interface of the Li metal anode in contact with various liquid electrolytes. By employing a non-washing protocol and a Cu current collector, we successfully captured and identified crucial reaction intermediates from SEI chemistry, enabling precise determination of the decomposition pathway for FSI anions. Our discoveries unveil a dynamic scenario at the electrode/electrolyte interface, where these reaction byproducts can significantly dissolve into the liquid layer, and their relative solubility dictates their role in passivating the electrode surface. Furthermore, we demonstrate a strong correlation between faster interfacial passivation in higher-performing electrolytes and the reduced solubility of diverse inorganic species within the liquid electrolyte. These fresh insights into SEI chemistry and formation open new avenues for enhancing LMA performance by mitigating undesired side reactions at the interface.