Improved electrochromic performance and lithium diffusion coefficient in three-dimensionally ordered macroporous V2O5 films

Abstract

Three-dimensionally ordered macroporous (3DOM) vanadium pentoxide (V₂O₅) films with various pore diameters were prepared by anodic deposition into colloidal crystal templates. The influence of the 3DOM structure on lithium ion (Li⁺) diffusion coefficient was investigated for the first time. X-ray diffraction analysis and HRTEM measurements show that the skeleton walls are composed of crystallites and amorphous V₂O₅. The study of electrochromic properties indicates that the pore size has a significant impact on the electrochromic performance. Small pores in the film lead to higher optical contrast and faster switching response. A high transmittance modulation in the visible and near-infrared spectral regions (50% at λ = 650 nm and 47% at λ = 900 nm) with fast response time (1.7 s for coloration and 3.2 s for bleaching) is observed in the 3DOM V₂O₅ film with a pore size of 210 nm. Because of the fully interconnected macroporous network in the 3DOM structure, the transport and reaction of lithium ions and electrons both behave in an effective 3D model throughout the whole nanostructure. Additionally, due to their influence on the polarization of the electrode and surface defects, sharp nanoscale edges around pores and rough surfaces can further promote Li⁺ diffusion and intercalation/de-intercalation. The 3DOM V₂O₅ film with a pore size of 210 nm exhibits a very high Li⁺ diffusion coefficient of 3.78 × 10⁻⁹ cm² s⁻¹, which is higher than any coefficient ever reported for a V₂O₅ film.