All-climate aqueous supercapacitor enabled by a deep eutectic solvent electrolyte based on salt hydrate

Aqueous supercapacitors (SCs) have received considerable attention owing to the utilization of low-cost, non-flammable, and low-toxicity aqueous electrolytes thus could eliminate the safety and cost concerns, but their wide temperature range applications have generally suffered from frozen of electrolyte and insufficient ionic conductivity at low temperatures. Herein, we demonstrate the feasibility of using an unconventional Deep Eutectic Solvent (DES) based on H2O-Mg(ClO4)2·6H2O binary system as electrolyte to construct all-climate aqueous carbon-based SC. This unconventional class DES completely base on inorganic substances and achieving simply mix inexpensive salts and water together at the right proportions. Attributed to the attractive feature of extremely low freeze temperature of −69 °C, this electrolyte can enable the 1.8 V carbon-based SC to fully work at −40 °C with outstanding cycling stability. This DES electrolyte comprising of a single salt and a single solvent without any additive will open up an avenue for developing simple and green electrolytes to construct all-climate SC.