Breaking the Underestimated Vicious Cycle of OH^- Migration via a Janus Separator in Aqueous Zn-Halogen Batteries

The anode-cathode crosstalk, driven by bilateral OH^- and polyhalide shuttling, forms an underestimated vicious cycle that critically degrades the capacity of aqueous Zn halogen batteries (AZHBs). In this cycle, anode corrosion produces OH^-, whose migration exacerbates cathode decay and polyhalide shuttling, which in turn leads to corrosion and increased OH^- production at the anode. To break such a self-amplifying degradation cycle, we develop a Janus separator functionalized with a porous ZIF-8 material encapsulated with benzotriazole (BTA) inhibitor. This layer is placed on the cathode side, where its ordered channels physically confine polyhalide shuttling, preventing corrosive species from reaching the Zn anode. Concurrently, BTA can be sustainably released from ZIF-8 framework, forming a stable solid-electrolyte interphase on Zn anode to isolate water-involved side reactions. By blocking polyhalide crossover and eliminating water-induced OH^- supply, this Janus design severs the OH^--driven vicious cycle at its source, without requiring additional electrolyte additives or complex anode modifications. Consequently, Zn||I₂ and Zn||TBABr₃ batteries incorporating the Janus separator achieve a three-fold extension in cycle life compared to batteries with a conventional separator, confirming that interrupting OH^- migration effectively halts the autocatalytic degradation pathway. Breaking this previously underestimated degradation mechanism provides a new route toward high-stability AZHBs.