Binary ionic liquid electrolyte design for ultrahigh-energy density graphene-based supercapacitors

Although room temperature ionic liquids (ILs) have emerged as potential next-generation electrolytes for their wide electrochemical stability window (ESW), the trade-off between this window and viscosity has hindered their widespread use in energy storage devices. Here, we present for the first time that such a trade-off can be balanced by mixing two ILs with the common anion ([NTf₂]⁻) but different cations ([EMIM]⁺ and [N1114]⁺) together. The [EMIM] cation-based IL possesses low viscosity while the [N1114] cation-based IL exhibits wide ESW. Since the concentrations of each IL in the mixtures can result in different electrolyte properties, we demonstrate a systematic approach by exploring the properties of various concentration combinations. In addition, the corresponding cell voltage of their resulting graphene supercapacitors (SCs) accompanied based on the interaction between the binary ionic liquid and the electrodes, and the associated electrochemical performance were studied to determine the optimum electrolyte system for the highest SC energy density. The well-balanced viscosity/ESW trade-off is achieved in binary IL consisting 50 vol% [EMIM][NTf₂] and 50 vol% [N1114][NTf₂] as evident from the extraordinary electrode specific capacitance of 293.1 F g⁻¹ and the ultrahigh SC energy density of 177 Wh kg⁻¹, which approaches that of a lithium-ion battery.