In this report, W⁶⁺ doping as a defect engineering strategy has been proposed to improve the electrochromic properties of NiO film. Further research was conducted to explore the electrochromic properties and the modified mechanism of W-doped NiO film. Compared to the pure NiO, W-doped NiO film exhibits improved electrochromic properties with significant optical modulation (61.56% at 550 nm), fast switching speed (4.42 s/1.40 s for coloring/bleaching), high coloration efficiency (45.41 cm²·C⁻¹) and outstanding cycling stability (no significant attenuation after 2000 cycles) in Li-based electrolytes. Density functional theory (DFT) calculations combined with the experimental results indicate that the improved electrochromic properties were due to enhanced the electronic conductivity and ion conductivity after the introduction of W⁶⁺. The charge capacity of W-doped NiO has also been improved, and it can function with WO₃ to achieve a high performance black electrochromic smart window (ECSW) by balancing charge. This work could advance the fundamental understanding of defect engineering as an effective strategy to boost the electrochromic properties of NiO anodic material, manifesting a significant development as a candidate counter electrode in high-performance black smart windows.

The energy consumption in building ventilation, air, and heating conditioning systems, accounts for about 25% of the overall energy consumption in modern society. Therefore, cutting carbon emissions and reducing energy consumption is a growing priority in building construction. Electrochromic devices (ECDs) are considered to be a highly promising energy-saving technology, due to their simple structure, active control, and low energy input characteristics. At present, H⁺, OH^- and Li⁺ are the main electrolyte ions used for ECDs. However, H⁺ and OH^- based electrolytes have a high erosive effect on the material surface and have a relatively short lifetime. Li⁺-based electrolytes are limited due to their high cost and safety concerns. In this study, inspired by prior research on Ca²⁺ batteries and supercapacitors, CaF₂ films were prepared by electron beam evaporation as a Ca²⁺-based electrolyte layer to construct ECDs. The structure, morphology, and optical properties of CaF₂ films were characterized. ECDs with the structure of ITO (indium tin oxide) glass/WO₃/CaF₂/NiO/ITO show short switching times (22.8 s for the coloring process, 2.8 s for the bleaching process). Additionally, optical modulation of the ECDs is about 38.8% at 750 nm. These findings indicate that Ca²⁺ based ECDs have the potential to become a competitive and attractive choice for large-scale commercial smart windows.

Electrochromic devices (ECDs) can regulate the indoor solar radiation by adjusting optical transmissive properties, showing great commercial potential and important social value of green energy saving. However, the unsafety and high cost of Li⁺ based electrolyte hinder the large-scale and industrialized production of ECDs. Other metal ions have been used as electrolyte ions, but they are rarely reported in all solid state ECDs. In this study, MgF₂ film is used as the solid electrolyte to construct all solid state ECD with the structure of glass/ITO/WO₃/MgF₂/NiO/ITO. The ECD shows the large optical modulation (~83% at 820 nm, with 100 s durations) and fast response (19.2 s for bleaching and 8.3 s for coloring, with 25 s durations). Moreover, the ECD achieves the extreme transmittance value of colored states Tc ≈ 0%, which can give an absolute private state. This work not only indicates that MgF₂ film can be an alternative to Li⁺ based electrolyte in all solid state ECDs, but also broadens the applications of all solid state EC smart windows to private buildings.