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

Abstract

Lithium (Li) metal negative electrodes have enticed a wide attention for high-energy-density batteries. However, its 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 been still controversial. In this work, we used 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 Li electrode potential (E_Li with reference to the redox of ferrocene) and solid electrolyte interphase (SEI) formation. Although the E_Li shows a different trend in DEE as compared to conventional 1,2-dimethoxyethane (DME), which is accounted for by different ion-pairing states of Li⁺ and FSI^-, the E_Li-CE plots are overlapped for both electrolytes, suggesting that the 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 FSI^- reduction potential, as demonstrated both experimentally and theoretically, which promotes FSI^--derived inorganic SEI. Both E_Li and SEI contribute to increasing the Li plating/stripping CE.