Direct Fabrication of LiCoO2 Films on Various Substrates in Flowing Aqueous Solutions at 150°C

In this paper, a newly developed idea for direct fabrication of double oxide films on various substrates, the “dual anode system,” is described. Films of LiCoO2 were directly fabricated on Pt, Ni, and graphite substrates by a hydrothermal–electrochemical method at 125–175°C for 2 hours in a flow cell using a cobalt metal anode as a cobalt ion source and a 4M LiOH aqueous solution as a lithium source, without any subsequent firing or annealing. The target substrate (Pt, Ni, or graphite) was also connected to the anode in the dual anode system. The addition of an oxidant, H2O2, in the flowing LiOH solution has produced LiCoO2 films in a short reaction time of 2 h using the flow cell. The H2O2 seems to accelerate the oxidation of Co2+ into Co3+ in cobalt species during the reactions, dissolution–oxidation–precipitation. The thickness and the microstructure of the films could be controlled. In optimum conditions, the film consisting of plate-like LiCoO2 crystals at most a few micrometers thick could be obtained. Micro-Raman and X-ray diffraction studies demonstrated that increasing the fabrication temperature produces a phase change in LiCoO2 from spinel to hexagonal. Spinel phase LiCoO2 is obtained around 125°C and hexagonal phase appears above 125°C. Above 150°C, the redissolution rate of formed crystals seems to dominate the formation of crystals; thus the grain size decreases with increasing temperature.