Effect of 20°-200°C fabrication temperature on microstructure of hydrothermally prepared LiCoO2 films

Films of LiCoO2 were directly fabricated by hydrothermal treatment of a cobalt metal plate in a 4M LiOH aqueous solution at 20 degrees -200 degreesC, with no subsequent annealing, and the effect of fabrication temperature on the film microstructure was investigated for the films. Micro-Raman studies have indicated that increasing the fabrication temperature produces a phase change in LiCoO2 from spinel to hexagonal. This change is revealed by a variation in the film thickness and the film surface morphology, as seen in the micrographs. The present scanning electron microscopy results showed a growth of spinel LiCoO2 particles up to 125 degreesC and the formation of hexagonal particles at >125 degreesC, in good agreement with the Raman and X-ray photoemission spectroscopy results. A film-formation mechanism based on the dissolution of cobalt metal, followed by precipitation, as LiCoO2, onto the cobalt substrate, is proposed. The mechanism is supported by experimental data, such as the one-step potential evolution (0 --> 0.6 V, with respect to the Ag/AgCl reference electrode) of the cobalt electrode during hydrothermal treatment and the detection of dissolved cobalt species by atomic absorption and ultraviolet-visible-light absorption spectroscopic analyses. Apparently, the evolution of the film structure arises from different nucleation and growth rates of LiCoO2 particles on the film, caused by the dissolution-precipitation mechanism, and a phase selection of spinel or hexagonal as the fabrication temperature increases.