Research On Electrochromic Properties Of Vanadium Pentoxide-Based Composite Films

Electrochromism is the reversible change of the optical properties（transmittance,reflectance,absorbance,etc.）of a material under the action of an applied alternating voltage.Among the transition metal oxides,vanadium oxide are the only electrochromic materials with anodic and cathodic coloring properties that has been found so far,which can achieve a blue-green-yellow color transition by ion insertion/extraction processes.In addition,vanadium pentoxide is receiving more and more attention because of its layered structure and large theoretical charge capacity(～294 m Ah g^-1).However,their poor electrical conductivity(～10^-2to 10^-3S cm^-1),low difusion coefcient of lithium ions(～10^-12to 10^-13cm²s^-1),and structural phase transitions during redox can cause lattice strain,leading to irreversible structural damage and ultimately irreversible capacity loss,which limits its practical application.Therefore,this paper aims to improve the electrochromic properties of vanadium pentoxide by preparing V₂O₅·n H₂O thin films,V₂O₅·n H₂O/（MXene）_xand V₂O₅·n H₂O/（GO）_xcomposite films by sol-gel and spin-coating methods to investigate the effects of monolayer MXene and monolayer graphene oxide doping on the structure,cycling stability,response rate and optical properties,as follows.（1）The interplanar distance can be calculated as 10.81（?）for the V₂O₅·nH₂O xerogel films.There are many wrinkles on the surface of the film,the root mean square roughness is 3.51 nm,the thickness is 128 nm.The cycling stability of V₂O₅·n H₂O xerogel films is poor（the charge capacity retention rate after 100 cycles is 75.05%）.The response rate of the V₂O₅·n H₂O xerogel film was slow（9.20 s for coloration and 7.20 s for decolorization）.The transmittance modulation of V₂O₅·n H₂O xerogel film at 547 nm is 11.41%.（2）After testing and characterization,it is found that MXene can increase the interplanar spacing of V₂O₅·n H₂O,increase the diffusion rate of lithium ions in V₂O₅·n H₂O thin films,and reduce the optical band gap.The V₂O₅·n H₂O/（MXene）_xcomposite films have high density and uniform surface.Due to the excellent mechanical properties and electrical conductivity of MXene,the cycling stability of V₂O₅·n H₂O thin films can be improved（when the MXene content is 4 wt%,the charge capacity retention rate of the composite films after 100 cycles can reach86.42%）,and the response rate can be improved.（When the MXene content is 3 wt%,the coloring time is 6.20 s,and the decolorizing time is 3.20 s）,and the optical modulation range is expanded（when the MXene content is 3 wt%,it is 49.03%at 372nm）.The V₂O₅·n H₂O/（MXene）_xcomposite films exhibited a light gray-green-yellow reversible color change.（3）Combining V₂O₅·nH₂O with GO did not destroy the layered structure of V₂O₅·nH₂O,but only increased the interlayer spacing,providing more space for the diffusion of lithium ions between films.The surface of the V₂O₅·n H₂O/（GO）_xcomposite films was smoother,because GO has excellent flexibility GO doping improves the cycling stability of V₂O₅·n H₂O films（When the GO content is 3 wt%,the charge capacity retention rate of the composite film after 100 cycles can reach84.48%）and significantly increases the response rate of V₂O₅·n H₂O films（coloration time of 5.8 s and bleaching time of 2.8 s at GO content of 5 wt%）.The V₂O₅·n H₂O/（GO）_xcomposite film showed a reversible color change of gray-brown-green-goose yellow,and GO led to an overall increase in the transmittance of the composite film,with an optical modulation rate of 35.34%at 1wt%GO content,and an increased optical modulation range in the near-infrared region for the V₂O₅·nH₂O/（GO）_xcomposite film mode.