Safe, Stable Cycling of Lithium Metal Batteries with Low-Viscosity, Fire-Retardant Locally Concentrated Ionic Liquid Electrolytes

Ionic liquid (IL) electrolytes with concentrated Li salt can ensure safe, high-performance Li metal batteries (LMBs) but suffer from high viscosity and poor ionic transport. A locally concentrated IL (LCIL) electrolyte with a non-solvating, fire-retardant hydrofluoroether (HFE) is presented. This rationally designed electrolyte employs lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), 1-methyl-1-propyl pyrrolidinium bis(fluorosulfonyl)imide (P13FSI) and 1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (TTE) as the IL and HFE, respectively (1:2:2 by mol). Adding TTE enables a Li-concentrated IL electrolyte with low viscosity and good separator wettability, facilitating Li-ion transport to the Li metal anode. The non-flammability of TTE contributes to excellent thermal stability. Furthermore, synergy between the dual (FSI/TFSI) anions in the LCIL electrolyte can help modify the solid electrolyte interphase, increasing Li Coulombic efficiency and decreasing dendritic Li deposition. LMBs (Li||LiCoO₂) employing the LCIL electrolyte exhibit good rate capability (≈89 mAh g⁻¹ at 1.8 mA cm⁻², room temperature) and long-term cycling (≈80% retention after 400 cycles).