Balancing the Co-Solvent Content in High Entropy Aqueous Electrolytes to Obtain 2.2 V Symmetric Supercapacitors

Abstract

The energy storage capability of supercapacitors (SCs) strongly depend on the operating cell voltage of the electrolytes of choice. In this regard, the inherent distinct electrochemical stability of cations and anions is a factor of relevance for the operating cell voltage. The use of double salts sharing one ion has been described as an approach to circumvent this problem, but whether modifying the solvation structure of cations and anions with different solvent molecules (coordinating and/or non-coordinating) could help balance their electrochemical stability in SCs has not yet been fully addressed. In this work, electrolytes are prepared by combining solvent mixtures and double salts, specifically 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF₄) in mixtures of water (H₂O) and dimethysulfoxide (DMSO) as coordinating co-solvents and acetonitrile (CH₃CN) as a weakly coordinating one. It is found that the presence of this latter one helped to enhanced the cation solvation structure (above 9). This increase of the entropic features allows operating at cell voltages of up to 2.2 V and the subsequent enhancement of the energy storage capabilities and capacitance retentions (up to 15 Wh kg⁻¹ and ≈87% after 10 000 cycles, respectively).