(13a) Understanding Confinement Effects in Mixed Ionic Liquid Systems: Insights from Molecular Dynamics Simulations

Molecular simulation has provided unique insights into the structure and dynamics of ionic liquids confined in microporous electrodes, which hold promise for their application as electrolytes in electrochemical capacitors, or supercapacitors. To date, however, most studied have relied on relatively simple and idealized systems, i.e. neat ionic liquids confined in single pores of regular geometry with no pore size dispersity. Real systems include any number of non-idealities, such as non-uniform pore sizes and other electrolytes or molecular species mixed in the fluid. Recent experimental results report complex behavior of mixtures of ionic liquids, with some beneficial and some detrimental features. Such results imply systems could be designed to exploit some of these features which perhaps mitigating some downsides. To this end, we investigate the possibility that ions may partition themselves in porous systems via their own molecular sizes. Using classical molecular dynamics of a system including the ionic liquids EMIM-Tf2N and HMIM-Tf2N in multiple porous electrodes of different sizes, we demonstrate that ions exhibit this partitioning behavior: smaller ions preferentially fill the smaller pores; likewise, larger ions preferentially fill the larger pores. The impacts of these findings on device performance, including rate handling capabilities, supercapacitor power density, and cycling capabilities, are discussed.