Local Superconcentration via Solvating Ionic Liquid Electrolytes for Safe 4.3 V Lithium Metal Batteries

A novel solvating ionic liquid (SIL), N-methyl N-oligo(ethylene oxide)pyrrolidinium bis(fluorosulfonyl)imide (Pyr1,(2O)7FSI) 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 Pyr1,(2O)7FSI) 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, Pyr1,(2O)7FSI: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.