Electric double-layer capacitors (EDLCs), also known as supercapacitors, are energy storage devices that deliver high power densities and long cycle lives but exhibit lower energy densities than Li-ion batteries. Improving the energy density of EDLCs without sacrificing their power density or lifetime is desirable for numerous energy storage applications. In order to rationally design higher energy and power density devices, an improved understanding of how electrolyte and electrode properties affect the performance of EDLCs is critical. Recently the use of ionic liquid mixtures as electrolytes has been experimentally reported to improve EDLC performance under certain conditions. However, existing theoretical and computational work on ionic liquid mixtures in EDLCs remains limited, and there remains a need for detailed understanding of how ionic liquid mixture electrolytes affect EDLC performance.
Using all-atom molecular dynamics simulations, we study the nanoscale adsorption behavior of mixtures of ionic liquids comprised of 1-ethyl-3-methylimidazolium (EMIM+), bis(trifluoromethylsulfonyl)imide (TFSI-), and tetrafluoroborate (BF4-) ions near planar carbon electrodes carrying various surface charges that mimic externally applied voltages. Near uncharged electrodes, we find that ion-electrode van der Waals interactions have a significant impact on the relative population of different ions in the first interfacial layer. By characterizing ion densities within a few nanometers of charged electrodes, we find that charging of the electrodes leads to greater overall changes in cumulative density of the smaller ion in each pure ionic liquid compared to the larger ion, as the larger ions tend to instead reorient and locally restructure close to the interface. Finally, in ionic liquid mixtures we identify an effective anion exchanging phenomenon near negatively charged electrodes that enhances counter-ion adsorption and provides a mechanism for capacitance enhancement. More broadly, our observation of modest capacitance differences between the ionic liquid compositions suggests that achieving substantially higher energy densities will ultimately also require tuning the structure and properties of the electrodes.
5:00 PM–7:00 PM Apr 23, 2019 (US - Arizona)
PCC North, 300 Level, Exhibit Hall C-E