Impact of Lithium Salt Addition to Ionic Liquid Electrolytes for High-performance Electric Double-layer Capacitors

Abstract

The charge-discharge behavior of electric double-layer capacitors (EDLCs) composed of several ionic liquids and their lithium solutions were evaluated. The model EDLC cell with a Li⁺-containing ionic liquid electrolyte, LiTFSI/EMImFSI (LiTFSI = lithium bis(trifluoromethylsulfonyl)imide, EMImFSI = 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide), exhibited a higher discharge capacitance than the cell containing EMImFSI, whereas the discharge capacitance of the cell with LiTFSI/EMImTFSI was lower than that of the cell containing EMImTFSI. In addition, the cells composed of EMImBF₄-based electrolytes displayed similar charge-discharge characteristics irrespective of the presence of the corresponding lithium salts. The distinctive results for the EDLCs with lithium-containing ionic liquid electrolytes are also discussed based on the application of the electrochemical impedance technique to model carbon electrodes in ionic liquids. According to the obtained differential capacitance of the model glassy carbon electrode in each ionic liquid electrolyte, the electric double-layer (compact layer) structure at the electrode/electrolyte interface depends on the component anion species of the ionic liquid and the co-existence of the anion and Li⁺.