Local superconcentration via solvating ionic liquid electrolytes for safe 4.3V lithium metal batteries

• Superconcentrated electrolytes were achieved with a solvating ionic liquid. • High performance electrolytes were formulated with the addition of hydrofluoroether. • Li metal deposition is improved vs. state-of-the-art ionic liquids. • Good electrochemical performance of NMC622||Li cells was achieved. A novel solvating ionic liquid (SIL), N -methyl N -oligo(ethylene oxide)pyrrolidinium bis(fluorosulfonyl)imide (Pyr 1,(2O)7 FSI) was synthesized and used to prepare binary and ternary liquid electrolytes with LiFSI as conducting salt and 1,1,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether (TTE) as a non-solvating low viscosity co-solvent. Thereby, the binary superconcentrated liquid electrolyte (i.e. , 6.8 m LiFSI in Pyr 1,(2O)7 FSI) reaches a Li + ion transference number of 0.25 ± 0.02. To enhance the ionic conductivity and separator wetting, the binary electrolyte was mixed with TTE leading to local superconcentrated Li + ion solvation structures as shown by Raman measurements. These ternary electrolytes exhibit improved wettability, excellent safety and allow cycling in NMC622||Li cells and Cu||Li cells with Coulombic efficiencies of up to 99.9% and 98.5%, respectively, and a capacity retention of 84% for NMC622||Li cells with the electrolyte 2.0 m LiFSI, Pyr 1,(2O)7 FSI:TTE (1:1 wt%) after 100 cycles vs. a cell failure after 35 cycles for the state-of-the-art containing IL N ‑butyl‑ N -methylpyrrolidinium FSI electrolyte.