Physical and Electrochemical Properties of Room Temperature Molten Salt Based on Aliphatic Onium Cations and Asymmetric Amide Anion

Room temperature molten salts (RTMS) have attracted attention as a novel safe electrolyte for electrochemical energy devices, such as lithium battery, because of various unique properties such as nonvolatility, incombustibility. We have studied the physical and electrochemical properties of RTMS based on aliphatic quaternary ammonium (AQA) and bis(trifluoromethylsulfonyl)imide (TFSI) systems because of their high cathodic stability comparing with imidazolium systems and high chemical stability in air. In AQA-TFSI melts, lithium deposition/stripping peak could be observed, however, AQA systems have several defects in view of battery electrolyte, such as. For example, they have higher viscosity, lower conductivity and higher melting point than imidazolium systems at the same molecular weight. Though TFSI anion has ability to lowering melting point of even in the symmetric aliphatic onium salts such as trialkylsulfonium salts, tetraalkylammonium salts, the melting point of small onium cation such as trimethylsulfonium, tetramethylsulfonium, which was expected to show low viscosity due to their small molecular weight, were no longer below room temperature. In this study, we proposed new anionic species with asymmetric structure (2,2,2-trifluoro-N-(trifluoromethylsulfonyl)acetamide, TSAC) to improve the melting point and the viscosity of small aliphatic onium cations. The physical and electrochemical properties of those new RTMS were reported comparing with corresponding TFSI salts.