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.