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

Although Li metal anodes offer the highest possible specific energy density for Li-based battery chemistries, practical application is plagued by Li filament growth, with adverse effects on both Coulombic efficiency (CE) and battery safety. The structure and resulting properties of the solid electrolyte interphase (SEI) on Li metal are 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 CE. This improved performance in the presence of LiNO3 is observed despite the fact that there are higher quantities and more types of compounds in the SEI than in LiTFSI alone, suggesting that the identity of the electrolyte decomposition products, rather than the amount, alters plating. SEI design strategies that increase SEI stability and Li interfacial exchange rate are thus expected to lead to more even current distribution, ultimately providing a new framework to generate smooth Li morphologies during plating/stripping.