Filmes amorfos de óxido de vanádio foram depositados sobre substratos de vidro recobertos com ITO, pela técnica de deposição de filmes de óxidos metálicos por filamento aquecido. Os espectros de difração de raios-X mostraram que os filmes como-depositados tratados termicamente em atmosfera de argônio em 200 e 300 o C continuaram amorfos e os tratados a 400 e 500 o C tornaram-se policristalinos, com a estrutura do V 2 O 5 . Os eletrodos foram eletroquimicamente reversíveis sob intercalação de Li + , exibindo o efeito eletrocrômico, observado por medidas da transmitância ótica em 632,8 nm. O coeficiente de difusão do Li, DC, foi medido pela técnica de titulação galvanostática intermitente (GITT) em função da carga inserida. Para os filmes cristalinos, a absorbância ótica e o DC aumentam com a quantidade de Li inserido, nas regiões de fase única de Li x V 2 O 5 cristalino e decrescem nas regiões de duas fases. Para essas, um DC efetivo foi considerado. Nos filmes cristalinos foi inferida a presença de outros óxidos de vanádio misturados à matriz de V 2 O 5 , através das medidas de cronopotenciometria e de DC.Amorphous vanadium oxide films were synthesized onto ITO-coated glass substrates by the hot filament metal oxide deposition technique. The as-deposited samples were heat-treated in an argon atmosphere. X-ray diffraction analysis revealed that the films treated at 200 and 300 o C were still amorphous, while those treated at 400 and 500 o C were crystalline, with a V 2 O 5 structure. All electrodes were electrochemically reversible for Li + intercalation, exhibiting the electrochromic effect, observed from optical transmittance measurements at 632.8 nm. The Li-diffusion coefficient, DC, was measured by the galvanostatic intermittent titration technique (GITT) as function of the inserted charge. For the crystalline films it was observed that the optical absorbance and the DC increase with increasing Li insertion in the single-phase regions of crystalline Li x V 2 O 5 and decrease in the two-phase regions. For the latter, an effective DC was considered. The presence of other vanadium oxides mixed to the V 2 O 5 matrix was inferred for the crystalline films from the chronopotentiometric and DC measurements.
Keywords: vanadium oxide film, chronopotentiometric, DC measurements
IntroductionThin films of vanadium oxides have been extensively studied as electrodes for lithium electrochemical insertion. Owing to the usually high charge intercalation capacity of these films, they are highly promising as cathodes for secondary microbatteries 1 and electrochromic devices. 2 The interest in these oxides lies on their performance as high Li intercalation capacity, high potential, high energy density, good cycling performance and on the unusual double electrochromic effect. The latter is the appearance of an anodic coloration in the near-ultraviolet region and a cathodic coloration in the near-infrared region as Li is intercalated. 3 Amorphous and crystalline vanadium oxide thin film electrodes have been prepared using chemical...