Neutral color and self-healable electrochromic system based on CuI/Cu and I₃⁻/I⁻ conversions

The electrochromic (EC) mechanisms of inorganic materials are usually based on reversible cation insertion/extraction or metal deposition/dissolution, which are plagued by ion trapping and dendrite growth, respectively. In this paper, a novel conversion-type electrochromic mechanism is proposed, by making good use of the CuI/Cu redox couple. This CuI-based electrochromic system shows a neutral color switching from transparent and dim grey. By simply increasing the bleaching voltage, I₃⁻/I⁻ redox couple can be further activated. The generated I₃⁻ will readily react with Cu, effectively improving the conversion reversibility and thereby rejuvenating the degraded electrochromic performance. Thanks to the well-designed electrode and the self-healing ability, this conversion electrochromic system achieves rapid response times (t_coloring: 23 s, t_bleaching: 6 s), decant optical modulation amplitude (26.4%), high coloration efficiency (86.15 cm²·C⁻¹), admirable cyclic durability (without performance degradation after 480 cycles) and excellent optical memory ability (transmittance variation < 1% after 10 h open-circuit storage). The establishment of this conversion-type electrochromism may inspire the exploration of novel electrochromic materials and devices.