Enhanced Li+ Transport in Ionic Liquid-Based Electrolytes Aided by Fluorinated Ethers for Highly Efficient Lithium Metal Batteries with Improved Rate Capability

FSI$^{-}$-based ionic liquids (ILs) are promising electrolyte candidates for long-life and safe lithium metal batteries (LMBs). However, their practical application is hindered by sluggish Li$^{+}$ transport at room temperature. Herein, it is shown that additions of bis(2,2,2-trifluoroethyl) ether (BTFE) to LiFSI-Pyr$_{14}$FSI ILs can effectively mitigate this shortcoming, while maintaining ILs′ high compatibility with lithium metal. Raman spectroscopy and small-angle X-ray scattering indicate that the promoted Li+ transport in the optimized electrolyte, [LiFSI]$_{3}$[Pyr$_{14}$FSI]$_{4}$[BTFE]$_{4}$ (Li$_{3}$Py$_{4}$BT$_{4}$), originates from the reduced solution viscosity and increased formation of Li$^{+}$-FSI$^{-}$ complexes, which are associated with the low viscosity and non-coordinating character of BTFE. As a result, Li/LiFePO$_{4}$ (LFP) cells using Li$_{3}$Py$_{4}$BT$_{4}$ electrolyte reach 150 mAh g$^{-1}$ at 1 C rate (1 mA cm$^{-2}$) and a capacity retention of 94.6% after 400 cycles, revealing better characteristics with respect to the cells employing the LiFSI-Pyr$_{14}$FSI (operate only a few cycles) and commercial carbonate (80% retention after only 218 cycles) electrolytes. A wide operating temperature (from −10 to 40 °C) of the Li/Li$_{3}$Py$_{4}$BT$_{4}$/LFP cells and a good compatibility of Li$_{3}$Py$_{4}$BT$_{4}$ with LiNi$_{0.5}$Mn$_{0.3}$Co$_{0.2}$O$_{2}$ (NMC532) are demonstrated also. The insight into the enhanced Li$^{+}$ transport and solid electrolyte interphase characteristics suggests valuable information to develop IL-based electrolytes for LMBs.