Fluorinated Carbonate‐aided Ionic‐liquid Electrolyte Enables High‐capacity Lithium‐metal Batteries

Abstract Ionic liquids (ILs) are promising electrolyte candidates for high‐safety and stability lithium metal batteries. However, their practical application is hindered by high viscosity resulting in a poor wettability of separator and sluggish Li + transport at room temperature. Herein, a weakly‐solvating solvent of methyl (2,2,2‐trifluoroethyl) carbonate (FEMC) is employed as a co‐solvent in non‐flammable ionic‐liquid electrolyte (ILE) which composed with N‐butyl‐N‐methylpyrrolidinium bis (fluorsulfonyl) imide (Pyr 14 FSI) and bis (trifluoromethanesulfonyl) sulfonimide lithium salt (LiTFSI), effectively ameliorates the shortcomings of ILs mentioned above without sacrificing its safety character. Compared with pure ILE, the addition of FEMC can distinctly decrease the polarization voltage and electrode‐electrolyte interface resistance. With the assistance of FEMC solvent, the discharge specific capacity of Li||LiFePO 4 (LFP) and Li||LiNi 0.6 Co 0.2 Mn 0.2 O 2 (NCM622) cells are significantly improved. The origin of this great improvement is contributed to the exceptionally stable and LiF‐rich cathode electrolyte interphase (CEI) layer formed in this electrolyte. Additionally, Li||LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) cells exhibit a remarkable rate capability (105 mAh g −1 at 2 C rate), achieve an initial specific capacity of 202.8 mAh g −1 and outstanding capacity retention of 88.2 % over 100 cycles.