Moisture-activated deep eutectic electrolyte enabling stable metal Zn anode

Abstract

Featuring cost-effectiveness, intrinsic safety and high energy density, rechargeable aqueous Zn-ion batteries employing metal Zn anodes have been considered as a crucial complement to the existing energy storage systems. However, the limited cycling reversibility of metal Zn anode in aqueous electrolyte hinders their further practical application, which is mainly resulted from the dendrite growth and water-induced parasitic reactions. Herein, a moisture-activated non-flammable deep eutectic electrolyte comprised of low concentrated ZnCl2 (0.21 m), choline chloride (ChCl) and ethylene glycol (EG) is prepared, in which Cl− and EG as well as water molecules are jointly participated in Zn2+ coordinating network. In contrast to conventional aqueous electrolytes and H2O-free ZADEE, the proposed Zn2+ solvation sheath structure ([ZnCl(EG)2(H2O)2]+) not only suppresses the water-induced side reactions and dendrite formation during Zn plating/stripping, but also decreases the Zn2+ de-solvation energy barrier and accelerates the Zn2+ transport. As a result, the dendrite-free Zn anode with long-term cycling lifespan (3000 h) and high average Zn plating/stripping Coulombic efficiency of 99.3% has been achieved. Furthermore, both the Zn-ion hybrid supercapacitors (ZIHSCs) and Zn-ion batteries (ZIBs) based on the ZADEE-4H2O electrolyte deliver an ultralong cycling lifespan (50000 cycles with 85.4% capacity retention for ZIHSCs and 10000 cycles with 71.4% capacity retention for ZIBs). Compared with the previous reported results, the proposed moisture-activated deep eutectic electrolyte can further facilitate the development of Zn-based energy storage devices.