Asymmetrical dicationic ionic liquids based on both imidazolium and aliphatic ammonium as potential electrolyte additives applied to lithium secondary batteries

Asymmetrical dicationic ionic liquids based on the combination of imidazolium and aliphatic ammonium cations with TFSI anion, MICnN111-TFSI2, have been synthesized for the first time, wherein MI represents imidazolium cation, N111 represents trimethylammonium cation, and Cn 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 ms cm−1, and electrochemical window of about 3.7 V at room temperature. To evaluate the potential of MIC5N111-TFSI2 as an additive of electrolyte for lithium secondary batteries, cells composed of LiMn2O4 cathode/1 M LiPF6 in EC:DMC (1:1, v/v) electrolytic solution containing 5 wt% 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.