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

Abstract

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-Pyr₁₄ 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 [Pyr₁₄ FSI]₄ [BTFE]₄ (Li₃ Py₄ BT₄ ), 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 Li₃ Py₄ BT₄ 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-Pyr₁₄ 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/Li₃ Py₄ BT₄ /LFP cells and a good compatibility of Li₃ Py₄ BT₄ with LiNi_0.5 Mn_0.3 Co_0.2 O₂ (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.