Solvation structure regulation of deep eutectic solvents: stabilization of the zinc anode in rechargeable zinc–air batteries

Abstract

Deep eutectic solvents (DESs) are promising electrolytes for the rechargeable zinc–air batteries (RZABs) but suffer from the low transport rate of Zn ligand ion, high over-potential of Zn redox, and dendrite growth of Zn metal. Here, we report a novel water-free ZnCl₂-based DES that achieves an effective regulation of the solvation structure of [ZnCl_x]^2−x by simply controlling the concentration of ZnCl₂. The coexisting solvation structure of [ZnCl₄]²⁻/[ZnCl₃]⁻ exhibits proper de-solvation energy and a relatively high transport rate of the zinc ligand ions. This unique solvation structure strikes a balance between the transport flux of [ZnCl_x]^2−x and the electrochemical reaction rate of [ZnCl_x]^2−x, as well as promotes Zn (002) homo-epitaxial deposition, enabling a highly reversible Zn plating/stripping with long-term stability of ∼1000 h at an ultralow over-potential of 33 mV. Furthermore, the RZABs with 1.0 M ZnCl₂ in DES demonstrated an impressive 8.7-fold enhancement in discharge performance (9.6 mA h cm⁻²) compared to the conventional alkaline electrolytes (1.1 mA h cm⁻²). This work demonstrates a new direction to produce a wide range of DES electrolytes with high ion conductivity and stable cycling performance.