Intrinsically Nonflammable Ionic Liquid‐Based Localized Highly Concentrated Electrolytes Enable High‐Performance Li‐Metal Batteries

The development of high‐performance Li‐metal batteries (LMBs) requires advanced electrolytes that simultaneously possess high safety, high ionic conductivity, wide electrochemical window, and good ability to suppress Li dendrite growth. Herein an intrinsically nonflammable ionic liquid‐based localized highly concentrated electrolyte (LHCE) composed of lithium bis(fluorosulfonyl)imide (LiFSI) salt, N‐methyl‐N‐propyl‐piperidinium bis(fluorosulfonyl)imide ([PP13][FSI]) as ionic liquid solvent, and 1,1,2,2‐tetrafluoroethyl‐2,2,3,3‐tetrafluoropropylether (HFE) as diluent solvent is designed. The introduction of HFE greatly decreases the viscosity and cost of the pure ionic liquid electrolyte, improves its ionic conductivity, and enhances its ability to wet the separator surface. The Li+ solvation structure, Li deposition behavior, and formation of the solid electrolyte interphase (SEI) layer in LHCE are systematically investigated by using Raman spectroscopy, theoretical simulations, scanning electron microscopy, and X‐ray photoelectron spectroscopy. A rational mechanism is suggested for the stable SEI formation and the homogeneous Li deposition behavior. Due to its excellent ability to suppress Li dendrites, the LHCE exhibits a high average Coulombic efficiency (99.4% over 800 cycles in the Cu/Li cell), extremely stable cycling performance (10 mA cm−2 over 5000 cycles in the Li/Li symmetric cell), and excellent cycling performance and rate capability in the LMB systems of LiFePO4 (LFP)/Li and LFP/Li@Cu. An intrinsically nonflammable localized highly concentrated electrolyte (LHCE) containing ionic liquid solvent (IL) and HFE diluent solvent is reported. The introduction of HFE greatly decreases the viscosity and cost of pure IL electrolyte, improves its ionic conductivity and its ability to wet the separator surface. Arising from homogeneous Li deposition behavior, the LHCE shows excellent cell performance in Li‐metal batteries.