Synchronous Enhancement of Ion and Electron Transport in Ce-Doped VNb₉O₂₅ toward Superior Electrochromic Performance

Wadsley-Roth phase niobium-based oxides have emerged as promising electrochromic candidates owing to their open 3D frameworks and excellent electrochemical stability. However, their practical application is still limited by intrinsically sluggish electrochemical kinetics. Herein, we demonstrate the electrochromic behavior of VNb₉O₂₅ for the first time and engineer its electrochemical kinetics via cerium (Ce) doping. At an optimal doping level, the resulting thin film exhibits more desirable performance characteristics. Structural characterizations by XRD and TEM confirm the Ce-induced lattice expansion, which facilitates ion diffusion. Meanwhile, Ce doping promotes the formation of oxygen vacancies within the lattice, enabling more efficient charge transport. Electrochemical studies manifest a notable decrease in charge transfer resistance and an increase in Li⁺ diffusion coefficient. Density functional theory calculations further confirm that the introduction of Ce modulates the electronic band structure and enhances the conductivity, providing a consistent mechanistic explanation for the accelerated electrochromic kinetics. As a result, the optimized Ce-doped VNb₉O₂₅ film achieves rapid switching speed, with coloring/bleaching time of 4.2/5.0 s at 600 nm, significantly faster than the undoped film (8.3/9.2 s). Moreover, it exhibits a large optical modulation (75.75% at 600 nm and 91.24% at 1100 nm) and retains more than 80% of its initial optical modulation after 5000 cycles. This work underscores the considerable potential of VNb₉O₂₅ for electrochromic applications and presents a facile doping strategy to enhance its electrochemical kinetics.