Highly Concentrated Aqueous Electrolyte with a Large Stable Potential Window for Electrochemical Double-Layer Capacitors

Abstract

Although there has been a major effort, all over the world, to increase an energy density of electrochemical capacitors (ECs) to meet more demands for electric automotive and regenerative energy storage applications; till now how to significantly increase the energy density of ECs is still the biggest challenge facing ECs. In the EC research, much efforts have been focused on the development of electrode materials with high specific capacitance (Cs) in various electrolytes, and on the design of various types of EC systems including battery-like electrode/capacitor-like electrode (hybrid capacitors) [1, 2], and recently reported alkali metal ion capacitors (Li ion battery-like capacitors) [3, 4]. Electrochemical capacitor systems based on active carbon (AC) can offer the advantages of low cost and environmentally benignity, together with a double-layer (DL) mechanism-based high power and super long cyclic life, and thus are expected to provide a various applications to meet ever-increasing energy demands. However, the obvious shortage of low intrinsic energy density makes an impediment of their widely exploration. With a growing interest recently in Lithium bis(trifluoromethane sulfonyl) imide ((LiN(SO2CF3)2, LiTFSI) as conducting salt for Li (or Li-ion) batteries[5, 6], in which a highly concentrated aqueous electrolyte (LiTFSI, i.e., water-in-salt), with expanded potential stable window of ~3.0 volts was reported. To utilize such extended potential window, if it is acceptable or suitable for EDLCs, would be much interesting and profoundly for the improvement of the performance of EDLCs; more obviously the energy density stored in EDLCs would be improved significantly, because the energy density of EDLCs increases quadratically with voltage. Driven by this consideration, in this paper, we try to demonstrate an AC/AC capacitor utilizing LiTFS or LiTFSI aqueous electrolyte. The potential window of such aqueous electrolytes with various salt concentrations is experimentally examined for single AC electrode and AC/AC coin cell, and the performance index of the AC/AC capacitor is also discussed. Fig.1 CV curves of an active carbon electrode (AC) behaviors completely capacitive in both LiTFS and LiTFSI aqueous electrolytes at 100 mV∙s-1(a, b); and an AC/AC coin cell in 20 M LiTFS (a, b) and 20 M LiTFSI aqueous electrolytes at 100 mV∙s-1(c). The experimental results shown in Fig.1 indicate that both LiTFS and LiTFSI aqueous electrolytes have similar electrochemical potential stable window for active carbon electrode, achieving -1.2 – 1.2 V vs.SCE, which overpass that of most traditional aqueous electrolytes. An AC/AC electrochemical capacitor in 20 M LiTFS or LiTFSI aqueous electrolyte present a stable working voltage range of 0 – 2.5 V, which is the largest voltage range for AC-based capacitor in aqueous electrolyte till now.