Revealing the Mechanism of Ni Vacancies on Ion Storage, Diffusion Kinetics, and Electrochromic Performance of Nickel Oxide Electrodes

Solar radiation can be dynamically regulated by the electrochromic smart window, showing great potential in reducing building energy consumption. As one of the most paramount inorganic electrochromic materials, NiO remains the elusive intricacies of the electrochromic process, particularly regarding the functional mechanisms of defects. Here, the mechanism of Ni vacancies (VNi) on the ionic storage, diffusion kinetics, and electrochromic performance are systematically investigated. The active VNi provides ion storage sites and promotes the transport of lithium ion (Li+) by forming diffusion channels, increasing the ion storage capacity from 2.12 to 4.90 mC cm-2. By introducing sub-gap absorptions, VNi increases the NiO's optical modulation from 14.4% to 30.3% with excellent cyclic stability (retain 87.4% after 4000 cycles). In contrast, the nonactive VNi barely contributes to the ion storage and electrochromic performance because of a high diffusion energy barrier, and causing low transmittance in the bleached state. Furthermore, VNi enables electrochromic devices to exhibit superior transmittance modulation and response speed, but nonactive VNi negatively affects the transparency in the bleached state. These results provide valuable guidance for building high-quality NiO electrodes through defect engineering and developing high-performing NiO-based electrochromic devices.