Composite Film of Vanadium Dioxide Nanoparticles and Ionic Liquid–Nickel–Chlorine Complexes with Excellent Visible Thermochromic Performance

Vanadium dioxide (VO 2 ), as a typical thermochromic material used in smart windows, is always limited by its weaker solar regulation efficiency (Δ T sol ) and lower luminous transmittance ( T lum ). Except for common approaches such as doping, coating, and special structure, compositing is another effective method. The macroscopic thermochromic (from colorless to blue) ionic liquid–nickel–chlorine (IL–Ni–Cl) complexes are selected in this paper to be combined with VO 2 nanoparticles forming a composite film. This novel scheme demonstrates outstanding optical properties: Δ T sol = 26.45% and T lum,l = 66.44%, T lum,h = 43.93%. Besides, the addition of the IL–Ni–Cl complexes endows the film with an obvious color change from light brown to dark green as temperature rises. This splendid visible thermochromic performance makes the composite film superior in function exhibiting and application of smart windows.