Dielectric‐Tailored Water Structure in Sepiolite‐Based Electrolyte for Stable Quasi‐Solid Zinc‑Iodine Batteries

Aqueous zinc‑iodine (Zn‐I 2 ) batteries face severe self‑discharge and anode instability due to polyiodide shuttle and side reactions, closely linked to the aqueous environment and elusive ionic behavior. The proposed electrolyte strategies for this issue focus on the high dielectric constant of water, ignoring the dielectric loss associated with water molecular polarization. Herein, we report a sepiolite‐based quasi‐solid electrolyte featuring a precisely tailored water structure including zeolitic, coordinated, and structural water, enabling fine‐tuning of dielectric properties and ion‐transport pathways. High zeolitic water content elevates the dielectric constant, along with the increased dielectric loss caused by dipole polarization of water molecules. Conversely, complete removal of water deteriorates Zn 2+ transport kinetics. The optimized electrolyte (ZnSEP‐2H 2 O) achieves an ideal balance between ion conductivity and polarization loss, effectively suppressing both Zn dendrite growth and polyiodide shuttle. As a result, Zn||Zn symmetric cells exhibit stable cycling over 2250 h at 0.4 mA cm − 2 , while Zn||AC@I 2 full cells retain a specific capacity of 100.7 mAh g − 1 after 50 000 cycles at 10 A g − 1 , with significantly mitigated self‐discharge. This work provides a rational electrolyte design strategy through water structure engineering and elucidates the governed mechanism of Zn 2+ migration kinetics and polyiodide confinement toward durable Zn‐I 2 batteries.