Hydrofluorocarbon electrolytes for energy-dense and low-temperature batteries

Electrolyte solvents for electrochemical devices have been dominated by oxygen (O)-based and nitrogen (N)-based ligands over the past decades^1-5, for which the dipole-ion (Li⁺, Na⁺ and so on) interaction usually lays the foundations of ion dissociation and transport but frustrates the charge transfer process at the electrolyte-electrode interface^6-9. Here, by synthesizing alkanes with monofluorinated structures, we show that fluorine (F)-based ligands with designed steric hindrance and Lewis basicity enable salt dissolution of more than 2 mol l^-1. Among them, 1,3-difluoro-propane (DFP)-based Li-ion electrolyte is endowed with all merits for energy-dense and low-temperature batteries, including low viscosity (0.95 cp), high oxidation stability (>4.9 V) and ionic conductivity of 0.29 mS cm^-1 at -70 °C. By incorporating F atoms in the first solvation shell, the weak F-Li⁺ coordination facilitates the Li plating/stripping process with Coulombic efficiency (CE) up to 99.7% and exchange current density one magnitude larger than O-Li⁺ coordination at -50 °C. The electrolytes further enable the operation of lithium-metal pouch cells under an electrolyte amount of less than 0.5 g Ah^-1, achieving energy densities greater than 700 Wh kg^-1 at room temperature and about 400 Wh kg^-1 at -50 °C. The hydrofluorocarbon (HFC) electrolytes in this work provide a feasible approach to building electrochemical systems beyond traditional coordination chemistry.