Ionic liquid-electrode interface at saturation: To crowd, or not to crowd?

At electrified interfaces, ions from concentrated electrolytes are known to arrange into alternating layers near highly charged surfaces. Saturation of such layers leads to a power-law decay in capacitance-potential curves. Some researchers relate this power-law to the crowding of ions; however, in this letter, we demonstrate that it can also result from the phenomenon known as overscreening. To help researchers answer the "to crowd or not to crowd" question, by distinguishing between these two charging regimes and ionic layer saturation, we derive an asymptotic description and examine molecular dynamics simulations, in which overscreening consistently dominates, while crowding is observed only under unrealistically extreme conditions. This insight expands understanding of how saturation works in concentrated electrolytes. Moreover, recognizing which mechanism is at play – crowding or overscreening – allows more precise control of charging behavior in electrochemical devices, from actuators, batteries, and capacitors to desalination systems, electrolyzers, and fuel cells.