Design of Symmetric TiO2/LiI/TiO2 Electrochromic Devices for Gradient Shaded Smart Windows with Enhanced Switching and Cycling Properties

Li+ insertion and extraction inherently impact the switching dynamics and cycling stability of inorganic electrochromic (EC) electrodes. Herein, the extraction of Li+ out of the TiO2 lattice to release the electron is replaced by the charge recombination of the electron with I-3(-) at the TiO2-electrolyte interface, which avoids the mechanical breakdown of the electrodes and renders self-bleaching with no applied voltages. A device design of the same two TiO2 nanocrystal (NC) electrodes combined with the redox lithium salt (LiI) electrolyte confers symmetric electrochromism. By applying a forward or reverse bias, the two TiO2 electrodes alternately serve as the electrochromic electrode and exhibit voltage-controlled gradient coloration, a maximum optical modulation of 90% at 700 nm, and a doubled cycling performance. The microstructure of the TiO2 NC film is characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Brunauer-Emmett-Teller methods. The electrochemical and electrochromic properties of the device are investigated using cyclic voltammetry, chronoamperometry, electrochemical impedance spectroscopy, and Mott-Schottky method.

Automated summary

Li+ insertion and extraction have significant effects on the switching dynamics and cycling stability of inorganic electrochromic electrodes. The extraction of Li+ from the TiO2 lattice is replaced by charge recombination with I-3(-) at the TiO2-electrolyte interface, allowing for self-bleaching without applied voltages. A device with two TiO2 nanocrystal electrodes and a redox lithium salt electrolyte demonstrates symmetric electrochromism and voltage-controlled gradient coloration.