Localized Dilution Enables Solvation-Controlled Ion Transport in Ultrathick Zn–I₂ Gel Batteries for Long-Duration Energy Storage

Thick electrodes are critical for long-duration energy storage, but their practical development is hindered by sluggish ion transport and elevated viscosity. Here, we present a solvation-based design strategy—Localized Diluted Electrolytes (LDEs)—that decouples ionic mobility from bulk viscosity through spatially engineered solvation heterogeneity. By promoting the nanoscale preferential distribution of salt-rich and water-rich domains, the LDE approach forms interconnected water-rich regions that sustain fluid-like ionic transport within a mechanically constrained gel matrix. Implemented in a zinc-iodine (Zn-I2) polymeric framework gel battery, this design enables a 2-mm-thick monolithic gel electrode with an areal capacity of ~ 28 mAh/cm² and an energy density of ~ 240 Wh/L based on cathode materials (~ 153 Wh/L for a full cell), achieving stable long-duration cycling. Nuclear magnetic resonance (NMR) spectroscopy, molecular dynamics simulations, and electrochemical analyses confirm rapid, sustained water and ion mobility throughout the polymer network, despite ~ 1200-fold increase in viscosity compared to the liquid-phase electrolyte. This work redefines how solvation environments can be spatially structured to decouple ionic transport from viscosity-driven limitations, establishing a new framework for electrolyte design.