Non‑metallic iodine is transparent in its reduced state and becomes opaque upon oxidation, thus possessing the intrinsic attributes required for electrochromic materials. However, its electrochromic application has long been hindered by a thermodynamic competition between deposited iodine species (including solid I₂ and insoluble polyiodides) and soluble migrating species such as I₃⁻. In conventional electrolytes, I₂ readily reacts with I⁻ to form stable soluble I₃⁻, shifting the pathway from deposition to dissolution and preventing reversible coloration.
To overcome this, we employ electrolyte engineering and interfacial regulation to redirect iodine conversion from dissolution diffusion to reversible deposition and conversion, as illustrated in Figure 1. First, a high-concentration ZnCl₂ water-in-salt electrolyte is designed to reconstruct the solvation sheath and suppress I₂ to I₃⁻ conversion, enabling reversible solid I₂ deposition with an optical contrast of 76.0%. Second, the introduction of [N₁₁₁₁]⁺ cations into the electrolyte induces confined solid-state I₅⁻ deposition, yielding 84.9% modulation retention after 20,000 cycles, a coloration efficiency of 49.8 cm² C⁻¹, and uniform large-area switching. Third, the introduction of MPI⁺ cations forms a liquid complex with I₃⁻, significantly enhancing kinetics and overcoming loading limitations of conventional electrochromic batteries, achieving an areal capacity of 56,396 mAh m⁻², far surpassing conventional devices (100-300 mAh m⁻²), while integrating electrochromic and energy storage functions.
 

Iodine,Non‑metallic,Reversible Electrodeposition