(709f) Solvation of Ionic Liquids on Supercapacitor’s Performance: Insights from Molecular Dynamics Simulation

Ionic liquids (IL), because of their good conductivities and exceptional stabilities, have attracted great attention as electrolytes for supercapacitors. Thanks to their wide electrochemical window, the energy density of supercapacitors has been improved greatly. While extensive researches have been conducted to study the electrical double layers (EDL) to investigate the charge storage mechanisms of supercapacitors, less care has been paid to the electrolyte dynamics in IL-based supercapacitors. There are concerns that the slow dynamics of ionic liquids may comprise the power density of supercapacitors. In practice, organic solvents are often added to improve the dynamic performance of ILs. Thus, a comprehensive study about the effects of solvation of IL on both energy density and power density of supercapacitors is needed.

In this work, we rely on molecular dynamics (MD) simulations to investigate 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([Emim][Tf₂N]) solvated in acetonitrile (ACN) at various carbon electrode surfaces, including pristine graphene, oxidized graphene, graphene edge, carbon onion and slit pores. For each electrode, we obtain capacitance as a function of ACN weight percentage, ranging from pure IL to pure ACN. Structure of electrical double layers is also analyzed and compared to bulk IL structures and between different electrode surfaces. To investigate the effect of solvation on the power density of supercapacitors, the ion self-diffusion coefficients are analyzed. Ion diffusivities in the solid-liquid interface are generally improved by adding more ACN, but different electrode surfaces exhibit slightly different behaviors. Combining both analysis of capacitance and ion dynamics, we are be able to comprehensively understand the effects of solvation of IL on supercapacitor’s performance.