Effect of 20°–200 ^o C Fabrication Temperature on Microstructure of Hydrothermally Prepared LiCoO ₂ Films

Abstract

Films of LiCoO 2 were directly fabricated by hydrothermal treatment of a cobalt metal plate in a 4 M LiOH aqueous solution at 20°–200°C, 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 LiCoO 2 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 LiCoO 2 particles up to 125°C and the formation of hexagonal particles at >125°C, 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 LiCoO 2 , 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 LiCoO 2 particles on the film, caused by the dissolution–precipitation mechanism, and a phase selection of spinel or hexagonal as the fabrication temperature increases.