Diverse Optical Regulations in a Single Electrochromic Layer of V₂O₅ via an Intrinsic Multiple Phase Transition

Electrochromic oxides possessing the characteristics of color variation and spectra modulation are desirable for smart windows, displays, and camouflage. Here, it is reported that, upon ion intercalation, a single layer of a model phase-transition material, V₂O₅, can possess diverse modulation of visible and near-infrared spectra while changing color, via intrinsic multiple phase transitions. Specifically, as the phase transitions from α to δ occur, the band transitions around 2.94 eV (420 nm) weaken, causing a blue shift in the optical absorption edge within the visible region. Simultaneously, new band transitions at 1.21 eV emerge and intensify, leading to a broad optical absorption centered around 1025 nm. As the phase transition progresses from δ to γ, the split-off bands in the γ phase fall below the Fermi level, which leads to a near-infrared transparent nature of γ-Li_xV₂O₅. Both absorption edge shifting and dynamically modulating the transmittance in V₂O₅ are different from other cathodic electrochromic oxides. The color-changing characteristics, together with selective spectral modulation, inspire the realization of multiple working modes for smart windows. Moreover, the optical constants of refractive index (n) and extinction coefficient (k) at various phases for Li_xV₂O₅ are also demonstrated. It is anticipated that multiple and reversible phase transitions, which have not yet been realized, will be the key design principle for achieving superior electrochromic devices.