Asymmetric Ionic Liquids for Enhanced Performance of Nanoporous Electrical Double Layer Capacitors

Real electrolytes are inherently asymmetric─ions differ in size, charge, and how they interact with each other and with electrode surfaces. However, most theoretical treatments still rely on idealized symmetric models, leaving gaps in our understanding of how real-world ionic asymmetry shapes electrolyte behavior. Atomistic simulations capture this complexity, but without a unifying picture, the field lacks clear design rules. This challenge is especially relevant for room-temperature ionic liquids, whose immense chemical tunability offers a powerful─but still underexploited─path to engineering high-performance electrical double-layer capacitors (EDLCs). In this work, we provide a systematic molecular-simulation study that reveals how asymmetries in ionic size, charge, and interactions govern the structure, dynamics, and energy-storage performance of conducting micropores representative of EDLC electrodes. Our results reveal striking phenomena that symmetric models overlook entirely, such as co-ion adsorption into nanopores at voltages for which co-ions would normally be expelled. Even more importantly, we demonstrate that ionic asymmetry can substantially boost energy-storage capacity. However, this enhanced capacitance introduces a key trade-off: increased energy storage comes alongside slower ion dynamics, imposing limits on power performance. These findings provide molecular-level insight into how asymmetry controls both the benefits and the constraints of real electrolytes. They also highlight a promising design strategy for next-generation EDLCs: leveraging controlled ionic asymmetry to push energy densities higher while carefully managing transport limitations.