Spontaneous reactions have been observed between LiCoO2 and electrolyte solvent for lithium ion batteries by characterizing the structural and/or components of the generated species in their gas, liquid and solid states. It is shown that surface coating cannot prevent the dissolution of Li and Co ions from LiCoO2 or the structural degradation of LiCoO2. However, surface modification can suppress the loss of oxygen from LiCoO2. Based on these results, another important aspect of the performance improvement mechanism by surface modification to LiCoO2 and the limitation of surface modification are proposed. (c) 2005 Elsevier B.V. All rights reserved.