Gradient Janus Hydrogel Electrolyte Regulating Interfacial Ion Transport for Simultaneous Cathode Utilization and Zn Anode Stability in Flexible Batteries

Aqueous zinc metal batteries (AZMBs) require electrolytes that reconcile the opposing roles of water at the cathode and zinc (Zn) metal anode. While sufficient water promotes cathodic reaction kinetics and capacity, excessive water activity triggers parasitic reactions and electrochemically inactive byproducts at the Zn anode. However, conventional homogeneous gel polymer electrolytes (GPEs) fail to satisfy these conflicting interfacial requirements. Here, we report one-step gradient-structured Janus gel electrolyte, achieved by dissolving the hydrophobic monomer acrylonitrile, with very low miscibility in water, into an aqueous electrolyte using the Hofmeister effect. Using two substrates with contrasting polar mismatches, this strategy yields a Janus GPE with asymmetric hydrophilicity and a gradient porous structure. When implemented in AZMB pouch cells, the Janus GPE delivers 97.7% capacity retention after 250 cycles in the pristine state and 92.6% retention relative to the pristine initial capacity over 400 cycles under folding. This performance stems from its asymmetric architecture: the cathode-facing hydrophilic surface provides a water-rich pathway for Zn²⁺ transport and cathodic kinetics, while the Zn-facing hydrophobic surface lowers water activity, suppresses parasitic reactions, and preserves interfacial integrity during bending. Overall, Janus GPE establishes a bend-resilient interfacial design for flexible AZMBs, simultaneously advancing cathode utilization and Zn anode stability.