Harnessing interfacial kosmotropic effect for reversible quasi-solid-state zinc-iodine batteries

Simultaneously achieving fast kinetics and interfacial stability remains a formidable challenge for quasi-solid-state zinc-iodine batteries. Herein, we introduce the interfacial kosmotropic effect via a low-cost CaSO₄·2H₂O ionic matrix to actively reconstruct the interfacial chemistry. Through the precise regulation of the electrical double layer, the strong electrostatic fields from surface-bound kosmotropic ions organize limited water molecules into a low-activity, confined network, fundamentally reshaping the solvation structure. Benefiting from this optimized atomic-level architecture, the reaction pathways are fundamentally altered: on the cathode, it unlocks a single-step I^-/I₂ redox process that effectively severs the formation pathway of polyiodides; on the anode, it induces coordinatively unsaturated structures to accelerate desolvation and suppress parasitic reactions. Consequently, the system circumvents intrinsic kinetic limitations and facilitates a transition from electric-field-driven electromigration to regulated diffusion-dominated transport. This optimizes ion flux and enables stable Zn anode operation for >1000 h (99.7% avg. Coulombic efficiency). The Zn-I₂ full cells deliver a high areal capacity of 0.91 mAh cm^-2 with 93% capacity retention after 2500 cycles at 1.0 mA cm^-2. Impressively, this strategy enables a minimalist iodine-free cathode design and successfully scales to pouch cells (stable for >600 cycles), offering a paradigm shift for developing safe, high-rate and durable quasi-solid-state batteries.