Studies on storage characteristics of LiNi0.4Co0.2Mn0.4O2 as cathode materials in lithium-ion batteries

LiNi0.4Co0.2Mn0.4O2 prepared by mixed-hydroxide method has shown excellent electrochemical performance and storage stability as a cathode material in lithium-ion batteries. The changes of the crystal structure, oxygen species on the surface, and oxidation state of transition metals of the samples under various storing conditions were examined by X-ray diffraction, iodometric titration, X-ray photoelectron spectroscopy, and the depth-profile experiments. It was found that the surface reaction between samples and CO2/H2O was so negligible that it could not affect the electrochemical performance of the material. But if storing the material in Ar-filled glove box, the oxygen might release from the surface, and, consequently, Mn and Co ions could decrease their oxidation state, take part in the electrode reaction, and then dissolve into electrolyte during the cycling process. The regular ion arrangement on the surface of the materials might be destroyed. Thus cyclic performance of the samples stored in Ar is poorer than that for samples stored in other conditions. (c) 2007 The Electrochemical Society.