One step synthesis of lamellar R-3m LiCoO2 thin films by an electrochemical-hydrothermal method

Abstract : In this study, we report the synthesis of lamellar R-3m LiCoO2 thin films electrodes for lithium rechargeable batteries by a single step method based on an electrochemical-hydrothermal synthesis in a concentrated LiOH solution with a cobalt salt. This process combines the effect of temperature (between 150 °C and 200 °C), pressure and galvanostatic current. The obtained films were not annealed after the electrochemical-hydrothermal synthesis. For the first time, the theoretical study of the potential-pH diagram of cobalt was carried out at high temperature and high concentration. These calculations show that a pH value higher than 12 is necessary to avoid the direct precipitation of cobalt hydroxide Co(OH)2 inside the solution. An improvement of the soluble species stability with an increase of the temperature and a decrease of the cobalt concentration is predicted. The influence of the deposition conditions (temperature and concentration) at a constant current density was experimentally studied. X-ray diffraction (XRD) shows the formation of well-crystallized LiCoO2 thin films. Raman spectroscopy confirmed the achievement of the electrochemically active R-3m LiCoO2 phase without any trace of the Fd3m phase at temperatures as low as 150 °C. Electrochemical measurements demonstrate good performances of the material synthesized between 150 °C and 200 °C with better capacity retention at higher temperature.