Enhancing Lithium‐Metal Battery Safety and Performance with Ether‐Aided Ionic Liquid Electrolytes

Electrolytes based on ionic liquids (ILs) are promising alternatives to conventional electrolyte systems due to their appealing properties, including enhanced safety owing to their high thermal stability and nonflammability, as well as their superior compatibility with lithium metal. However, their high viscosity limits Li-ion mobility. To overcome this challenge, we developed a dual-anion IL electrolyte incorporating an ether-type cosolvent. This electrolyte combines N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (PYR14FSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and 1,2-dimethoxyethane (DME). The incorporation of DME into the electrolyte, Pyr14FSI + LiTFSI 8:2 + x wt.% DME (x = 10, 20, 30, 40), results in an improved Li-ion mobility and promotes the formation of a more stable solid electrolyte interphase, facilitated by the DME-induced decomposition of the FSI− and TFSI− anions. Our findings suggest that the addition of 20 wt.% of DME allows for the optimum balance between improved transport properties owing to the presence of DME and high thermal stability thanks to the presence of the IL. This optimized electrolyte yields very stable long-term cycling of Li‖NCM622 cells, along with a substantially higher capacity compared to the neat IL electrolyte, thus providing a viable approach to overcome the remaining challenges of IL-based electrolytes, while maintaining their intrinsic advantages.