Low-Cost “Water in Deep-Eutectic-Solvent” and Eutectogel Electrolytes for Long-Life High-Voltage Zinc-Ion Hybrid Supercapacitors

Abstract

Zinc-ion hybrid supercapacitors (ZHSCs) with high voltage and long cycle life are promising. However, active water in most aqueous electrolytes limits the electrochemical stability window and promotes side reactions, resulting in an unsatisfactory cycle life. Herein, a novel low-cost “water in deep-eutectic-solvent” electrolyte composed of ZnAc2 and methylurea is reported to expand the electrochemical stability window to >2.7 V, allowing ZHSCs to possess high operating voltage. Benefiting from a water-starved solvation sheath, the zinc anode was protected from dendritic growth and side reactions by reducing the participation of H2O during zinc plating and 3D diffusion of zinc ions in the eutectic electrolyte. Therefore, the assembled Zn//ZM-30//N-rGO@Ti ZHSC displayed an excellent capacitance retention of 90.8% after constant current charge/discharge for 9300 cycles at 10 A g-1, while the Zn//ZM-30//Zn symmetric cells can work stably for more than 1100 h at 1 mA cm-2. Additionally, the ZHSC with ZM-30 also demonstrated a high power density of 4004.3 W kg-1 (41.6 Wh kg-1) at 8 A g-1 and a high energy density of 167.7 Wh kg-1 (462.5 W kg-1) at 0.5 A g-1. Next, a new eutectogel electrolyte was prepared and it perfectly inherited the advantages of liquid eutectic electrolytes, which kept 85.2% capacitance retention after 8000 cycles at 5 A g-1. Meanwhile, it obtained excellent mechanical properties and met the application of quasi-solid flexible energy storage devices under deformation conditions.

Keywords: Zinc-ion hybrid supercapacitors (ZHSCs), deep-eutectic-solvent, Electrochemical stability window, eutectogel.