Redox-active hybrid-solvent-in-salt electrolyte for high energy density supercapacitor

A major limitation of supercapacitors (SCs) is their lower energy density compared to batteries, despite their high rate capability and long cycle life. One promising approach to overcoming this challenge is the use of high-concentration electrolytes, such as water-in-salt electrolyte (WiSE) and its redox-active (RA) derivatives, which leverage redox reactions to enhance energy density. However, these systems are constrained by the low solubility of RA species, limiting further performance improvements. To address this, we introduce a novel redox-active aqueous/organic hybrid-solvent-in-salt electrolyte (RA-HiSE), which significantly improves the solubility of RA species. As a result, SCs utilizing RA-HiSE achieve a remarkable three-fold increase in energy density (251.2 W h kg⁻¹ at 2.5 A g⁻¹) compared to those with RA-WiSE, along with excellent capacity retention (95 % over 10,000 cycles at 5 A g⁻¹). In addition to enhancing energy density, the RA-HiSE system offers superior temperature stability compared to RA-WiSE. This development represents a key breakthrough in the advancement of next-generation electrolytes for high-energy-density SCs.