Confined Solid‐State Polyiodide Deposition Enables Durable Electrochromic Smart Windows

Electrochromic smart windows are promising for dynamic building energy management. Yet, conventional ion‐intercalation systems suffer from structural degradation, while reversible metal electrodeposition systems are limited by dendrite growth and poor durability. Here, we report a Zn‐coupled electrochemical device, enabled by coordination‐governed reversible polyiodide deposition. By introducing a highly symmetric tetramethylammonium cation ([N 1111 ] + ), precise spatial matching with linear I 5 − is achieved, inducing the formation of a highly stable solid‐state [N 1111 ] + I 5 − complex. This effectively suppresses the shuttle effect of polyiodides and enables highly reversible I − /I 5 − conversion. Meanwhile, [N 1111 ] + further regulates the solvation structure at the Zn electrode via interfacial adsorption, constructing a water‐deficient inner Helmholtz layer that promotes uniform Zn 2+ deposition. Benefiting from the synergistic stabilization of the solid‐state polyiodide deposition/dissolution and reversible Zn plating/stripping process, the assembled device delivers a high optical contrast of 75.5% at 650 nm, fast switching speeds (3.9/6.5 s), and 84.9% retention after 20 000 cycles. The device blocks 93.6% of solar irradiation in the colored state, enabling a 4°C–10°C cooling effect and 16.4% annual energy savings. This work establishes a new design paradigm for Zn‐coupled electrochromic devices based on confined solid‐state polyiodide chemistry, providing a promising strategy for constructing durable and scalable smart windows.