Elucidating synergistic mechanisms of an anion–cation electrolyte additive for ultra-stable zinc metal anodes

Abstract

Uncontrollable dendrite formation and rampant parasitic reactions in zinc (Zn) metal anodes obstruct the practical application of aqueous Zn–metal batteries (AZMBs). Herein, we demonstrate a synergistic cation–anion regulation strategy to stabilize Zn metal anodes using NaI as a proof-of-concept additive for the ZnSO₄ electrolyte system. By combining rigorous physicochemical, computational and electrochemical analyses, it is found that I⁻ anions can reshape the solvation sheath of Zn(H₂O)₆²⁺, break the association of H₂O and specifically absorb on the Zn surface, promoting Zn²⁺ transfer kinetics, guiding homogeneous Zn deposition and constraining parasitic reactions. Meanwhile, the Na⁺ cations absorb on irregular Zn tips as an electrostatic shielding to prevent dendrite growth. As a result, at an optimal additive concentration of 0.2 M, the Zn symmetric cells can deliver an astonishing 8632 h cyclability at 1 mA cm⁻²/1 mA h cm⁻², exceeding by 83-fold that obtained using BE electrolyte. Furthermore, the additive supports highly reversible Zn stripping/plating at −10 °C, enables Zn‖NaV₃O₈·1.5H₂O to attain significantly upgraded rate and cycling performances, and most importantly and uniquely, unlocks the high-capacity I⁻/I₃⁻ redox couple for long-cycling Zn‖I batteries. This work provides a novel strategy to stabilize Zn metal anodes towards dendrite-free AZMBs.