Decoding the gradient-distributed colour centers in electrochromic WO₃

The optical properties of electrochromic windows can be modulated by embedding electrons and ions simultaneously to realize energy-saving functions. However, the microscopic electrochromic mechanism in cathode materials remains controversial. Herein, we have discovered that the single-crystal WO₃ nanosheets exhibit thickness-dependent electrochromic characteristics. The multislice electron ptychography experiments revealed that the colour centers in electrochromic WO₃ are associated with lattice distortions, and the density of the colour centers decreases from surface to bulk. Moreover, the heterogeneous distribution of polarons induces a phase transition from monoclinic to tetragonal and cubic structures, which in turn causes the band gap of the WO₃ nanosheets to follow a volcano-type trend. This phenomenon elucidates the colour shift of the electrode from yellow to blue or black as the thickness of the nanosheets decreases. By visualizing the actual colour centers of electrochromic materials, our findings provide an optimal framework for the development of high-performance electrochromic devices.