New class of hydrophobic ionic liquids based on bis(2,2,2-trifluoroethoxysulfonyl)imide ([(CF3CH2OSO2)2N]−, TFESI−) anion with various oniums, including N,N-dialkylimidazolium, tetraalkyl (ether-containing alkyl) ammonium, N-alkyl-N-methyl-pyrrolidinium, and -piperidinium, are prepared and characterized. Their physicochemical and electrochemical properties, including phase transitions, thermal stability, viscosity, density, ionic conductivity and electrochemical windows, have been determined. The influences of the structural variations in both the cation and anion on the above properties are extensively discussed. The key features of these new TFESI−-based ionic liquids are their low melting points (ranging from 9 to 44°C) and/or low glass transition (between −74 and −61°C), relatively low viscosities (44–426cP at 25°C), good thermal stability (T d >270°C), and wide electrochemical windows (ranging from 5.7 to 6.4V).