Direct fabrication of lithium cobalt oxide 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 LiCoO₂were 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, H₂O₂in the flowing LiOH solution has produced LiCoO₂films in a short reaction time of 2 h using the flow cell. The H₂O₂seems 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 LiCoO₂crystals at most a few micrometers thick could be obtained. Micro-Raman and X-ray diffraction studies demonstrated that Inc.reasing the fabrication temperature produces a phase change in LiCoO₂from spinel to hexagonal. Spinel phase LiCoO₂is 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 Inc.reasing temperature.