Asymmetrical dicationic ionic liquids based on the combination of imidazolium and aliphatic ammonium cations with TFSI anion, MIC n N111-TFSI2, have been synthesized for the first time, wherein MI represents imidazolium cation, N111 represents trimethylammonium cation, and C n represents spacer length. The physical and electrochemical properties of this family of ionic liquids were studied. 1-(3-Methylimidazolium-1-yl)ethane-(trimethylammonium) bi[bis(trifluoromethane-sulfonyl) imide] (MIC2N111-TFSI2) shows solid–solid transition characteristics. 1-(3-Methylimidazolium-1-yl)pentane-(trimethylammonium) bi[bis(trifluoromethan-esulfonyl)imide] (MIC5N111-TFSI2) has one of the lowest solid–liquid transformation temperatures among analogues, and belongs to the greatest thermal stable ionic liquids. Additionally, it has an order of conductivity of 10−1 mscm−1, and electrochemical window of about 3.7V at room temperature. To evaluate the potential of MIC5N111-TFSI2 as an additive of electrolyte for lithium secondary batteries, cells composed of LiMn2O4 cathode/1M LiPF6 in EC:DMC (1:1, v/v) electrolytic solution containing 5wt% of MIC5N111-TFSI2/lithium metal anode have been prepared. The charge–discharge cycling test reveals that unlike the cases of Li/LiMn2O4 cells employing a conventional electrolyte with a monocationic ionic liquid, such as 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EtMeImTFSI) as an additive, the performances of Li/LiMn2O4 cells do not drop with the addition of MIC5N111-TFSI2 at 1C rate, moreover, the cell exhibits better discharge capacity and cycle durability compared with the cell using the conventional electrolyte.