Self-generated coating of LiCoO₂ by washing and heat treatment without coating precursors

The concept of a self-generated coating on LiCoO₂ that does not require any additional cost-consuming coating precursors has been proposed by a combination of simple processes, water washing and heat-treatment. Surface impurities, such as carbonate and hydroxide, on the LiCoO₂ surface are finely controlled during the self-generated coating process. The coating improves the electrochemical performance when both washing and heat-treatment are used, whereas each individual process is not effective. The surface changes of LiCoO₂ are analyzed to elucidate the mechanism of the self-generated coating using X-ray photoelectron spectroscopy, FT-IR spectroscopy, X-ray diffraction, and electron microscopies. The thickness of the coating layer and the type of the coating materials are controlled by the temperature of the heat-treatment after the washing process. When a high temperature of 700°C is used for the heat-treatment, a heavy coating layer having several tens of nm thickness is generated on the LiCoO₂ powder that severely interferes with the migration of Li ions and the electron conduction, thereby degrading cycle performance. On the other hand, the samples annealed at a low temperatures of 400°C has a thin coating layer with thickness of 10∼30 nm that improves the cycleability by inhibiting the electrolyte decomposition.