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-Pyr14FSI 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[Pyr14FSI]4[BTFE]4 (Li3Py4BT4), 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/LiFePO4 (LFP) cells using Li3Py4BT4 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-Pyr14FSI (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/Li3Py4BT4/LFP cells and a good compatibility of Li3Py4BT4 with LiNi0.5Mn0.3Co0.2O2 (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.