Ternary zinc-based deep eutectic solvents for durable zinc-ion capacitors capable of withstanding extreme high temperatures

Abstract

The employment of aqueous zinc salt solutions as electrolytes confers cost and safety advantages to aqueous zinc-ion capacitors (ZICs). However, the high chemical H₂O with low boiling point in typical aqueous zinc sulfate (ZS) electrolyte greatly restricts the performance of ZICs in high-temperature conditions. Herein, a ternary deep eutectic solvent (DES) electrolyte capable of withstanding extreme high temperatures of up to 80 °C is synthesized for the first time by utilizing inter-component hydrogen bonding. The DES with optimized component ratios allows long-term stable cycling of Zn||Zn cells for >1200 h with nearly 100 % Coulombic efficiency. When used as the high-temperature electrolyte, ZICs at 80 °C yield a much higher specific capacitance of up to 247 F g⁻¹ than at room temperature, while those with ZS as the electrolyte hardly work properly under this condition. The hydrogen bonding system reconfigured by ligand competition in DESs effectively inhibits Zn dendrite formation and the activity of H₂O, thus endowing ZICs up to 10000 cycles at 80 °C. This ternary DES not only opens the way for the design of high-temperature electrolytes for ZICs, but also offers new possibilities for the development of other energy storage devices that need to be adapted to extreme operating environments.