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

Abstract

AbstractCyano‐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 (Pyr14)‐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.