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

Abstract

Asymmetrical dicationic ionic liquids based on the combination of imidazolium and aliphatic ammonium cations with TFSI anion, MIC n N 111-TFSI 2, have been synthesized for the first time, wherein MI represents imidazolium cation, N 111 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] (MIC 2N 111-TFSI 2) shows solid–solid transition characteristics. 1-(3-Methylimidazolium-1-yl)pentane-(trimethylammonium) bi[bis(trifluoromethan-esulfonyl)imide] (MIC 5N 111-TFSI 2) 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 MIC 5N 111-TFSI 2 as an additive of electrolyte for lithium secondary batteries, cells composed of LiMn 2O 4 cathode/1 M LiPF 6 in EC:DMC (1:1, v/v) electrolytic solution containing 5 wt% of MIC 5N 111-TFSI 2/lithium metal anode have been prepared. The charge–discharge cycling test reveals that unlike the cases of Li/LiMn 2O 4 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/LiMn 2O 4 cells do not drop with the addition of MIC 5N 111-TFSI 2 at 1 C rate, moreover, the cell exhibits better discharge capacity and cycle durability compared with the cell using the conventional electrolyte.