Surface materials on positive electrodes of lithium-ion batteries which have been degraded are characterized by ATR technique of FT-IR. Lithium carbonate, P=O bond based material, lithium alkyl carbonate (ROCO2Li), and polycarbonate-type compound are detected which have been formed by decomposition reactions of electrolyte solvent and LiPF6 as lithium salt. Stability of lithium carbonate for charging to SOC = 100% is depended on the condition during degradation. Especially, it changes to unstable by long time cycling at low temperatures. Formation of polycarbonate-type compound during cycling is remarkable at elevated temperatures. Degradation of batteries is larger in higher temperatures, and it is suggested that the variation of surface structure has relation to degradation of batteries. (c) 2007 Elsevier B.V. All rights reserved.