Halogenated Zn²⁺ Solvation Structure for Reversible Zn Metal Batteries

Rechargeable aqueous Zn metal batteries have become promising candidates for large-scale electrochemical energy storage owing to their high safety and affordable low cost. However, Zn metal anode suffers from dendritic growth and hydrogen evolution reaction (HER), deteriorating the electrochemical performance. Here, we demonstrate that these challenges can be conquered by introducing a halogen ion into the Zn²⁺ solvation structure. By designing an electrolyte composed of zinc acetate and ammonium halide, the electron-donating anion I⁻ can coordinate with Zn²⁺ and transform the traditional Zn(H₂O)₆²⁺ to ZnI(H₂O)₅⁺, in which I⁻ could transfer electrons into H₂O and thus suppress HER. The dynamic electrostatic shielding layer formed by concomitant NH₄⁺ can restrict the dendritic growth. As a result, the halogenated electrolyte achieves a high initial coulombic efficiency (CE) of 99.3% in the Zn plating/stripping process and remains at an average of ∼99.8% with uniform Zn deposition. Moreover, Zn-I batteries are constructed by using dissociative I⁻ as the cathode and carbon felt-polyaniline as the conductive and adsorptive layer, exhibiting an average CE of 98.6% without capacity decay after 300 cycles. This work provides insights into the halogenated Zn²⁺ solvation structure and offers a general electrolyte design strategy for achieving a highly reversible Zn metal anode and batteries.