Synergistic cathode-anode modulation via a multifunctional ionic liquid additive for high-performance zinc‑iodine...

Aqueous zinc‑iodine batteries (ZIBs) hold great promise for large-scale energy storage owing to their high safety and low cost. However, their commercial viability is severely challenged by dendrite growth and hydrogen evolution at the Zn anode, as well as the polyiodide shuttle effect at the iodine cathode. Herein, we introduce a multifunctional ionic liquid additive, 1-hydroxyethyl-3-methylimidazolium trifluoromethylsulfonate ([HEMIM][OTf]), which leverages the synergistic effect of its cation and anion to enhance the stability of both electrodes simultaneously. Combined computational and experimental investigations reveal that the anion (OTf^-) can participate in the Zn²⁺ solvation sheath, thereby modulating its coordination structure and suppressing side reactions. Concurrently, the cation (HEMIM⁺) preferentially adsorbs onto the Zn anode surface, promoting the formation of a water-lean inner Helmholtz layer that further mitigates hydrogen evolution and corrosion. Moreover, OTf^- undergoes in situ reduction at the anode surface to form a ZnF₂-rich protective layer, which inhibits dendrite growth and facilitates uniform Zn deposition. As a result, Zn║Zn symmetric cells exhibit excellent rate capability and long-term cycling stability. On the cathode side, HEMIM⁺ effectively anchors I₃^- through electrostatic interactions, significantly suppressing polyiodide shuttling and improving active material utilization. Consequently, the Zn║I₂ full cell delivers a reversible capacity of 120.5 mAh g^-1 after 4100 cycles at a high current density of 2.0 A g^-1, with a capacity retention of 90.5%, demonstrating outstanding cycling performance. This work provides a new strategy for the synergistic interfacial optimization of both electrodes in ZIBs through the design of electrolyte additives.