Stabilization of lithium metal in concentrated electrolytes: effects of electrode potential and solid electrolyte...

Lithium (Li) metal negative electrodes have attracted wide attention for high-energy-density batteries. However, their low coulombic efficiency (CE) due to parasitic electrolyte reduction has been an alarming concern. Concentrated electrolytes are one of the promising concepts that can stabilize the Li metal/electrolyte interface, thus increasing the CE; however, its mechanism has remained controversial. In this work, we used a combination of LiN(SO₂F)₂ (LiFSI) and weakly solvating 1,2-diethoxyethane (DEE) as a model electrolyte to study how its liquid structure changes upon increasing salt concentration and how it is linked to the Li plating/stripping CE. Based on previous works, we focused on the Li electrode potential (E_Li with reference to the redox potential of ferrocene) and solid-electrolyte-interphase (SEI) formation. Although E_Li shows a different trend with DEE compared to conventional 1,2-dimethoxyethane (DME), which is accounted for by different ion-pair states of Li⁺ and FSI^-, the E_Li-CE plots overlap for both electrolytes, suggesting that E_Li is one of the dominant factors of the CE. On the other hand, the extensive ion pairing results in the upward shift of the FSI^- reduction potential, as demonstrated both experimentally and theoretically, which promotes the FSI^--derived inorganic SEI. Both E_Li and SEI contribute to increasing the Li plating/stripping CE.