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 ([N₁₁₁₁]⁺), precise spatial matching with linear I₅ ^- is achieved, inducing the formation of a highly stable solid-state [N₁₁₁₁]⁺I₅ ^- complex. This effectively suppresses the shuttle effect of polyiodides and enables highly reversible I^-/I₅ ^- conversion. Meanwhile, [N₁₁₁₁]⁺ further regulates the solvation structure at the Zn electrode via interfacial adsorption, constructing a water-deficient inner Helmholtz layer that promotes uniform Zn²⁺ 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.