High Voltage Imidazole-Based Dicationic Ionic Liquid Organic Electrolytes for Electrochemical Double Layer Capacitors

Abstract

Since expanding the operating voltage window is the most effective way to increase the energy density of electrochemical double layer capacitors (EDLCs), the development of novel ionic liquid electrolytes with good chemical stability has been a research focus in this field. In this work, dicationic ionic liquids (DILs) with highly adjustable structure are selected as the research object, and a series of high purity imidazole-based DILs (CnIM.BF4 and CnIM.TFSI, n = 2, 4, 6, and 10) with different lengths of alkyl bridging are synthesized by changing the interlocking groups between the two cations. Using propylene carbonate (PC) as the solvent, the physical and electrochemical performances of all DILs/PC electrolytes are measured and evaluated systematically. The results show that the DILs exhibit higher electrochemical stability than monocationic ionic liquids (MILs), and the operational voltages of EDLCs using DILs/PC electrolyte are all above 3.0 V, which are significantly higher than that of MILs/PC electrolyte system (2.8 V). Among them, the C4IM.BF4/PC electrolyte exhibits the smallest solvent resistance, the highest capacitance, the widest operating voltage window (3.2 V) and excellent cycling performance. As a result, the energy density of EDLCs using C4IM.BF4/PC electrolyte is approximately 33% higher than that of using MILs electrolytes. In general, this work provides a highly competitive DILs electrolyte with high voltage and long cycling life.

Keywords: Dicationic Ionic liquids, electrolyte, electrochemical double layer capacitor, potential window, energy density