Nonfluorinated Ionic Liquid Electrolytes for Lithium Metal Batteries: Ionic Conduction, Electrochemistry, and Interphase Formation

Cyano-based ionic liquids (ILs) are prime candidates for the manufacturing of cheaper and safer batteries due to their inherently low-volatility and absence of expensive fluorinated species. In this work, N-methyl-N-butylpyrrolidinium (Pyr(14))-based ILs featuring two different cyano-based anions, i.e., dicyanamide (DCA) and tricyanomethanide (TCM), and their mixture with the respective Li salts (1:9 salt:IL mole ratio), alongside their combination (DCA-TCM), are evaluated as potential electrolytes for lithium metal batteries (LMBs). The electrolytes display significant ionic conductivity at room temperature (5 mS cm(-1)) alongside an electrochemical stability window up to 4 V, suitable for low-voltage LMBs such as Li-sulfur, as well as promising cycling stability. In addition to the detailed physicochemical (viscosity, conductivity) and electrochemical (electrochemical stability window, stripping/plating, and impedance test in symmetrical Li cells) characterization, the solid electrolyte interphase (SEI) formed in this class of ionic liquids is studied for the first time. X-ray photoelectron spectroscopy (XPS) provides evidence for an SEI dominated by a polymer-rich layer including carbon-nitrogen single, double, and triple bonds, which provides high ionic conductivity and mechanical stability, leading to the aforementioned cycling stability. Finally, a molecular insight is achieved by density functional theory (DFT) and classic molecular dynamics simulations both supporting the experimental evidence.