High-Efficiency Lithium Metal Anode Enabled by a Concentrated/Fluorinated Ester Electrolyte.

: Lithium (Li) metal anode (LMA) has received growing attention due to its highest theoretical capacity (3860 mA h g-1) and the lowest redox potential (-3.04 V versus standard hydrogen electrode). However, the practical application of LMA is obstructed by the detrimental side reactions between Li metal and organic electrolytes, especially when cycled in traditional carbonate ester electrolytes. Herein, we propose a novel fluorinated carbonate ester-based electrolyte by combining diethyl fluorocarbonate (ETFEC) solvent and 5 M high LiFSI concentration (M=mol L-1). Using this electrolyte, an ultra-high Li plating/stripping Coulombic efficiency (CE) of 99.1% can be obtained in Li||Cu cells and a stable cycle performance of Li||LiFePO4 is achieved under the conditions of limited Li metal (5 mA h cm-2), moderate loading LiFePO4 (7~8 mg cm-2) and lean electrolyte (40 uL). The fundamental functioning mechanism of this novel electrolyte has been carefully investigated by scanning electronic microscopy (SEM), operando optical microscopy (OM), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and solid state nuclear magnetic resonance (SS-NMR). The results demonstrate that this optimized electrolyte facilitates the formation of a high Li+ conductive SEI layer enriched with LiF and inorganic sulfur-containing species, which can effectively suppress the side reactions between electrolyte and Li metal and prevent the formation of dead Li.