Interfacial Anion Activation for Durable Aqueous Zn-Ion Batteries

Designing a robust solid electrolyte interphase (SEI) is pivotal for stabilizing the Zn anode/electrolyte interface in aqueous Zn-ion batteries (AZIBs). However, the mechanism of SEI formation in AZIBs remains insufficiently understood, particularly when additives induce the formation of mixed inorganic and organic SEI components during electrochemical reactions. Herein, we demonstrate how an allylurea additive induces the formation of an organic-inorganic hybrid SEI (h-SEI) at the Zn anode interface in a ZnSO4-based electrolyte and ultimately leads to the improvement in long-term interfacial stability. Unlike conventional electrolyte designs based on contact ion-pair solvation formation, allylurea reconstructs the interfacial environment to establish an anion-rich, water-deficient region near the Zn surface, thereby promoting electric-field-driven reductive transformation of anions. In situ spectroscopic analyses and theoretical calculations indicate that the electrochemical activation of allylurea produces allyl cation intermediates, which mediate the stepwise conversion of sulfate anions into ZnS. The resulting ZnS-rich inorganic domains reinforce the interphase and facilitate Zn2+ transport, whereas allylurea-derived organic components enhance interfacial adhesion and mechanical compliance. Consequently, the h-SEI homogenizes Zn deposition and suppresses interfacial side reactions, enabling stable cycling of Zn||Zn symmetric cells for over 2500 h at 5 mA cm-2 and Zn||Na2V6O16·3H2O full cells for more than 10,000 cycles at 5 A g-1.