Anion-Rich Solvation Structures in High Entropy Aqueous Electrolytes for Supercapacitors with Superior Potential Windows and Rate Capabilities

The use of aqueous electrolytes as suitable electrolytes for supercapacitor (SCs) strongly depends on the opening of the operating cell voltage while preserving good rate capabilities so to obtain high energy and power densities over numerous cycles. Electrolyte engineering with anion- and/or solvent-rich solvation structures has been recently explored in batteries of any sort to enhance the interphase stability at high voltages, typically by the formation of a SEI. Less explored has been this approach in SCs to not only extending the range of operational voltage but also balancing 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 to widening the cell voltage at which the electrolyte is capable to operate (e.g., up to 2.2 V). Interestingly, this extremely-rich solvation structure also exhibited the best transport properties for the different ions that, ultimately, were responsible of an excellent maintainance of the energy density at high power densities.