Probing the Dynamics of Electric Double Layer Formation over Wide Time Scales (10^-9-10⁺⁵s) in the Ionic Liquid DEME-TFSI

We investigate the transient response of N,N-diethyl-N-(2-methoxyethyl)-N-methylammonium bis(trifluoromethylsulfonyl)-imide-based ionic liquid (IL) planar capacitors, studying this response over time scales ranging from as little as a few nanoseconds to as much as several days. Our measurements point to the existence of three distinct mechanisms for charging/discharging of the IL. The fastest of these is associated with the development of a standard polarization charge in the bulk of the liquid dielectric, which dominates at times less than 10-6 s. The second process is attributed to electric double layer formation, which is initiated after 10-6 but which takes as long as 10-2 s to reach completion. Finally, we also identify the presence of a pseudocapacitance that arises from electrochemical reactions; this process is only activated at voltages above 2.5 V and is relatively slow. Indeed, we find evidence that full discharging of this pseudocapacitance can take as long as 105 s (i.e., days). Overall, our findings provide useful insights into the mechanisms for slow ion dynamics in ILs and highlight the constraints that these dynamics place on the potential operational speed of IL-based transistors.