Strategic Synthesis of Mixed-Phase Tungsten Oxide for Electrochromic Smart Windows

Electrochromic devices based on mixed-phase WO₃ can potentially outperform their pure-phase counterparts due to the optimized distribution of optically active sites that facilitate cation intercalation. In this work, we synthesized WO₃ containing orthorhombic and hexagonal phases, with precise phase ratio control accomplished through a meticulously designed experimental strategy of optimizing the reaction time at low temperatures in a closed system under a hydrogen atmosphere. A detailed XRD analysis shows an optimal phase ratio (orthorhombic/hexagonal = 0.59), corresponding to a hexagonal content of 62.7% that demonstrated superior electrochromic performance. A fast Li⁺ ion diffusion (diffusion coefficient of 3.105 × 10^-10 cm²/s) indicated more optically active sites for ion intercalation, enabling high transmission modulation of 52%, excellent coloration efficiency of 133 cm²/C, and fast switching in less than 2.7 s. The presence of phase junctions significantly enhanced the structural stability up to 5000 cycles. The mixed-phase configuration stabilized the structural deformation during Li-ion interaction and intercalation, likely contributing to improved reversibility and, consequently, increased stability of the electrode. The multifunctional characteristics were elucidated by establishing the relationship between optical modulation, the charge storage capability, and the heat-blocking ability of the best-performing electrochromic device. Additionally, the material's synthesis and device fabrication protocol employed in this work yields a scalable, cost-effective, and stable dual-functional device suitable for the construction of smart windows.