Low-cost “water in deep-eutectic-solvent” and eutectogel electrolytes for high-performance 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 (DES)” electrolyte composed of Zn(Ac)₂ and methylurea (Mu) is reported to expand the electrochemical stability window to >2.7 V, significantly higher than common aqueous electrolytes. 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 (the mass of Zn(Ac)₂/Mu DES: the mass of water = 100: 30) //N-rGO@Ti ZHSC displayed 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 >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.