The Polarity of Co-solvents Regulates the Charge Storage Mechanisms in Supercapacitors with Concentrated Electrolytes.

Developing better energy storage devices depends on comprehending the underlying mechanisms involved in charge storage. With the continuous conception of new electrolytes, this task becomes progressively more urgent and complex. An example is the utilization of co-solvated concentrated solutions. While these show promising electrochemical responses, their dynamic properties (especially under confinement) and their relationships with performance are not fully understood. Here, we combined modified step potential electrochemical spectroscopy and quasielastic neutron scattering to investigate systems composed of activated mesoporous carbon (AMC) and concentrated solutions of lithium bis(trifluoromethanesulfonyl)imide in acetonitrile co-solvated with either toluene or acetone. We report that acetone does not impair surface-controlled mechanisms, contrary to the case with toluene, which competes with charged species to populate the AMC's pores without contributing to charge storage. In turn, toluene promotes a greater overall capacitance owing to Faradaic processes, which may be related to changes in the solvation structures under confinement.