Ionic Liquid Mixtures for Supercapacitor Applications: Synergy of Electrochemistry, NMR, and Simulations

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The ever-growing global energy demand requires developing novel energy-storage technologies to increase energy production from renewable sources and the transition from hydrocarbon-based fuel to electrical drive. Supercapacitors with nanoporous electrodes have emerged as a critical energy-storage technology, offering high power densities and remarkable cyclability. However, they provide only moderate energy densities compared with conventional batteries and limited electrochemical performances at low temperatures. Neat ionic liquids (ILs) as an electrolyte allow high operating voltages (>3.5 V) and boost the energy storage, but the reduced ion mobility lowers the power density.Increasing the energy storage without compromising the power density would make these ecologically-friendly energy storage systems more broadly applicable. Recently, few studies showed that mixing different ILs improves supercapacitor performance by extending the electrochemical window and temperature range. While these reports are encouraging, there is no deep understanding of the underlying physical mechanisms. Many critical questions remain unanswered: How do ion properties in a mixture affect the bulk and in-pore diffusivity of ions and ultimately the charging dynamics? How does the electrochemical performance vary with the temperature? How to match the pore size and ion sizes in a mixture to maximize the stored energy? What is the relationship between the carbon structure/pore sizes and ion kinetics during charging? These are just some of the key issues essential for further development in this field. Our project addresses these questions by combining electrochemistry, in-situ nuclear magnetic resonance (NMR) measurements, and molecular simulations. We will scrutinize how the electrochemical performance is correlated with the properties