<p>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 LiNO<sub>3</sub> + 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.</p>