Low-Cost “Water-in-Deep Eutectic Solvent” Electrolyte for High-Performance Zinc Ion Hybrid Supercapacitors

Zinc ion hybrid supercapacitors (ZIHSs) with low cost, high safety, and high capacity density are considered reliable alternatives for future energy applications. However, the poor cycle reversibility of Zn metal anodes in aqueous electrolytes hinders their further practical application, mainly due to dendrite growth and water-induced parasitic reactions. Herein, we have prepared a deep eutectic solvent (DES) by mixing ZnCl2 and urea and added a very small amount of water to optimize the utilization of this DES as an electrolyte for ZIHS. This DES has significantly suppressed the side reactions and dendrite growth in ZIHS, resulting in a wide electrochemical stability window of 2.3 V, a maximum energy density of 255.4 Wh kg–1 at a capacity density of 127.7 mAh g–1, and good cycling stability of ZIHS, maintaining 79.5% capacity density after 3000 cycles, and the Coulombic efficiency has been maintained at over 99%.