Hydrated eutectic electrolyte as catholyte enables high-performance redox flow batteries

Compared with conventional electrolytes, eutectic electrolytes are recognized as an ideal electrolyte for the nextgeneration electrochemical energy storage system characterized by facile synthesis and composition tunability. However, the eutectic electrolytes still suffer from sluggish kinetics, which inhibit their widespread application in redox flow batteries. Here, a well-designed hydrated eutectic electrolyte system (HEE) composed of iron chloride hexahydrate, urea and water with stoichiometric ratio at 2:1:6 is developed as catholyte to improve the cycling performance of hybrid redox flow batteries. The HEEs with precisely controlled water content could inhibit the water-induced side reactions, while expand the electrochemical stability windows. Moreover, the water molecules gradually replace the anions to participate in the solvation sheath of Fe3+ ions, forming a solvation structure of [FeClx(H2O)y]3-x, which result in an improved ion dissociation degree, ionic conductivity and redox kinetics. As a result, hybrid iron-based redox flow battery based on the optimal HEE exhibits a highly reversibly redox reaction of Fe3+/Fe2+ and reduced overpotentials. Furthermore, the battery could stably cycle over 120 cycles with a capacity retention of 87.75 % at a relatively high current density of 10 mA cm -2, delivering a maximum power density of -50 mW cm -2, which is much higher than reported eutectic-based flow batteries. This research offers new insights into rationally designing of advanced eutectic electrolytes for highefficient redox flow batteries.