Di-imidazolium ionic liquid based electrolytes for high voltage electrochemical double layer capacitors

The biggest limitation of the current electrochemical double layer capacitors (EDLCs) is the low energy density, one of the reasons is the poor electrochemical stability of the commercial TEABF4/acetonitrile electrolyte, resulting in the operating voltage window (OVW) of the device is only 2.7 V. Thus, the development of novel ionic liquid electrolytes with good chemical stability has been a research focus in this field. In this work, a series of high purity di-imidazolium ionic liquid (DILs) (Cndi[MIM].di[BF4] and Cndi[MIM].di[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. As a result, the OVW of EDLCs using DILs/PC electrolyte are all above 3.0 V, which are significantly higher than that of EDLCs using mono-imidazolium ionic liquids (MILs) based electrolyte (2.8 V for EMIM.BF4/PC). Among them, the EDLCs using C4di[MIM].di[BF4]/PC electrolyte exhibits the smallest solvent resistance, the highest capacitance, the widest operating voltage window (3.2 V) and excellent cycling performance, and improves the energy density by approximately 33 % compared to the EDLCs using EMIMBF4/PC electrolytes. In general, this work provides a highly competitive DILs electrolyte with high voltage and long cycling life.