A coarse-grained model of room-temperature ionic liquids between metal electrodes: a molecular dynamics study

Recent mean-field theories predict that room-temperature ionic liquid (RTIL) electric double-layer capacitors (EDLCs) undergo a spontaneous surface charge separation (SSCS) with no applied potential. In this study, we construct a coarse-grained molecular model that corresponds to the mean-field models to directly simulate the behavior of RTILs without invoking mean-field approximations. In addition to observing the SSCS transition, we highlight the importance of the image charge interactions and explore the enhanced in-plane ordering on the electrodes, two effects not accounted for by the mean-field theories. By calculating and comparing the differential capacitance for RTILs confined between perfectly conducting and non-metal electrodes, we show that the image charge interactions drastically improve the energy storage properties of RTIL EDLCs. Ion-ion correlations and image charge interactions in RTILs at metal interfaces result in a spontaneous surface charge separation, which is not seen with non-metal electrodes, suggesting the importance of the surface metallicity on RTIL EDLCs.