Charging Dynamics and Diffusion of Ionic Liquids in Defective Graphite-Based Supercapacitors

Recent advancements in supercapacitor technology have underscored the potential of graphite electrodes paired with ionic liquid electrolyte 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) for efficient energy storage. While significant research has explored ion transport mechanisms at the electrode–electrolyte interface, particularly in porous electrodes, the influence of surface imperfections, such as point defects and square hole-like defects, on the graphite electrode surface remains less understood. In this study, we employ molecular dynamics simulations to investigate how random point defects and square hole defects affect the charging dynamics and ion diffusion behavior in graphite-based supercapacitors. We analyze the impact of these surface imperfections on key performance metrics, including charging time, surface charge density, and ion diffusion under varying electric field and pressure. For pristine graphite, we observe a camel-shaped differential capacitance–voltage dependence, with reduced charging time and increased cross-plane ion diffusion coefficient as voltage increases. The diffusion coefficient saturating at an applied voltage of 1.5 V. Point defects have minimal effect on charging relaxation time and the surface charge density, though the differential capacitance peaks at a defect concentration of 1.2%. In contrast, square pore-like defects lead to increases in charge density with defect size and depth, while charging time and diffusion coefficient display more complex dependencies. These results offer insight into defect-mediated electrochemical behavior in graphite electrodes and suggest pathways for engineering improved supercapacitor performance.