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. ... mehrA 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.