Electrochromic devices constructed with water-in-salt electrolyte enabling energy-saving and prolonged optical memory effect

Electrochromism, a promising energy conversion technology, has shown enormous prospects in various energy-efficient applications. The novel Zn2+ based aqueous electrochromic system exhibits self-coloration and large transmittance modulation, while the performance is stagnated by the limited bleaching voltage causing poor kinetics of the reaction. To clear the obstacles of trapped zinc ions and short lifetime, the emerging water-in-salt (WIS) electrolytes of Zn2+ were introduced into tungsten oxide-based electrochromic devices for the first time. The ZnCl2 WIS electrolyte leads to an expanded electrochemical stability window up to 2.4 V by suppressing the electrolysis of water molecules. The electrochromic device with ZnCl2 WIS electrolyte shows enhanced performance of large transmittance modulation of 64% at 660 nm, a short response time (4.8/3.6 s for coloring/ bleaching), and high cyclic stability when compared with that of 2 M ZnCl2 aqueous electrolyte. The enhanced performance is further revealed by the theoretical analyses and numerical simulation of the ion transportation kinetics in the electrochromic layer. Moreover, a patterned electrochromic display is fabricated by a simple brush-painting method. The display utilizing ZnCl2 WIS electrolyte yields a prolonged optical memory effect compared to that with 2 M ZnCl2, where the content of this display is still visible even after three days without external energy consumption. This work presents good feasibility for electrolyte design strategy and provides an attractive electrolyte candidate for high-performance electrochromic applications.