Ultrahigh-voltage aqueous electrolyte for wide-temperature supercapacitors

Designing wide electrochemical stability window (ESW) aqueous electrolytes is essential to achieve high-voltage supercapacitors (SCs). A general solution using “water-in-salt” electrolytes is proposed. However, “water-in-salt” electrolytes possess inferior ESW, inhibiting their practical applications. Here, a low salt concentration (1 M Na₂SO₄ (mol kg_solvent⁻¹)) aqueous electrolyte using ethylene glycol (EG) as additive is reported. EG molecules strongly coordinate with Na⁺ ions and some H₂O molecules stay out of the Na⁺ solvated shell layer without participating in the decomposition process on the surface of an electrode, thus achieving an impressively wide ESW (3.88 V). An excellent coulombic efficiency of 95% with ultra-high capacitance retention of approximately 100% during 10 000 charge–discharge cycles under 3.2 V operating voltage was achieved. Benefiting from the low saturation of EG, there was only less than 5% decrement of capacitance retention under 3.0 V operating voltage at −40 °C. Moreover, the SCs achieve excellent capacitance retention of 98% after 5000 cycles even at 105 °C. This work provides a promising basis for developing novel, sustainable SCs.