Anion-rich solvation structures in high entropy aqueous electrolytes for supercapacitors with enlarged potential windows and superior rate capabilities

Abstract

The use of aqueous electrolytes as suitable electrolytes for supercapacitors (SCs) largely depends on maintaining a wide operating cell voltage window while ensuring good rate capabilities, thereby achieving high energy and power densities over many cycles. Electrolyte engineering with anion- and/or solvent-rich solvation structures has been recently explored in batteries of any type to enhance the interphase stability at high voltages, typically by the formation of a SEI. This approach has been less explored in SCs to not only extend the range of operational voltage but also balance the inherently distinct electrochemical stability of cations and anions with carbon electrodes. Herein, we prepared electrolytes composed of two ionic liquids with a common cation (e.g., EMIMTFSI and EMIMBF₄) in solvent mixtures of H₂O, DMSO and CH₃CN. We found that the electrolyte with the richest solvation structure (i.e., with many and diverse anions that surrounded the cation) was the most effective at widening the cell voltage at which the electrolyte is capable of operating (e.g., up to 2.2 V). Interestingly, this extremely rich solvation structure also exhibited the best transport properties for different ions that, ultimately, were responsible for an excellent maintenance of the energy density at high power densities.