Ionic liquids based on aliphatic tetraalkylammonium dications and TFSI anion as potential electrolytes

A new series of ionic liquids containing aliphatic tetraalkylammonium dications with alkyl linkage chains and TFSI anion as potential electrolytes were prepared and characterized. Some of the important physicochemical and electrochemical properties of these prepared ionic liquids, including melting point, density, solubility in common solvents, ionic conductivity, thermal and electrochemical stability, were investigated. These ammonium dicationic ionic liquids exhibited five types of phase transition behavior on heating and one stage decomposition behavior. The spacer length and the head group overall length were responsible for the solid/liquid transformation temperature, and the spacer length was more effective in lowering the solid/liquid transformation temperature than the head group overall length. The interesting findings for such dicationic ionic liquids was that they showed the lowest solid–liquid transformation temperatures among analogues with alkyl links, and belonged to the greatest thermal stable ionic liquids. The Arrhenius equation approximately described the relationship between conductivity and temperature for several prepared dicationic ionic liquids in the low temperature region studied. The electrochemical windows were about 4.3 V for C9(N222)2-TFSI2 and C12(N222)2-TFSI2, and 4.7 V for C6(N118)2-TFSI2 at room temperature.