Rapid Interfacial Exchange of Li Ions Dictates High Coulombic Efficiency in Li Metal Anodes

Although Li metal batteries offer the highest possible specific energy density, practical application is plagued by Li filament growth with adverse effects on both Coulombic efficiency and battery safety. The structure and resulting properties of the solid electrolyte interphase (SEI) on Li metal is critical to controlling Li deposition morphologies and achieving high efficiency batteries. In this report, we use a combination of nuclear magnetic resonance (NMR) spectroscopy and X-ray photoelectron spectroscopy (XPS) to show that fast Li transport and low solubility at the electrode/SEI interface in 0.5 M LiNO₃ + 0.5 M LiTFSI electrolyte bi-salt in 1,3-dioxolane:dimethoxyethane (DOL:DME, 1:1, v/v) are responsible for the formation of low surface area Li deposits and high Coulombic efficiency, despite the fact that the SEI is thicker and chemically more heterogeneous than LiTFSI alone. These data suggest that SEI design strategies that increase SEI stability and Li interfacial exchange rate will lead to more even current distribution, ultimately providing a new framework to generate smooth Li morphologies during plating/stripping.