Self-Generated Coating of LiCoO2 by Washing and Heat Treatment without Coating Precursors

Abstract

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.