Effects of Solvent Concentration on the Performance of Ionic-Liquid/Carbon Supercapacitors

We use molecular dynamics simulations to investigate the effects of solvent concentration on the bulk properties of an ion liquid electrolyte and the electrochemical performance on carbon-based electrodes, including pristine graphene, oxidized graphene, graphene armchair edge, graphene zigzag edge, onion-like carbon, and slit-pore carbon. We find that diluting the electrolyte reduces the number of ion pairs in the bulk and improves ion dynamics. The capacitance of the two-edge electrodes decreases monotonically as the solvent concentration increases, while the capacitance of other nonedge electrodes exhibits nonmonotonic behavior and a capacitance maximum is observed. Further analyses on the electric double layer reveals two competing factors: solvation reduces the charge overscreening effect, but it also causes the dilution of absorbed ion concentration. While the former increases the capacitance in the low dilution regime, the latter decreases the capacitance in the high dilution regime. In addition, the dilution also significantly improves the ion dynamics at the interface. Our simulation results demonstrate that diluting an ionic liquid electrolyte could potentially boost the power density while maintaining or even slightly increasing the energy density with a careful selection of solvent concentrations on a nonedge carbon electrode.