Lithium-ion batteries using LiNi0.8Co0.15Al0.05O2 (NCA) as the positive electrode material and hard carbon as the negative electrode material with electrolyte of mixture of ethylene carbonate and dimethyl carbonate containing LiPF6 were fabricated, and the surface materials on the positive electrode were observed by ATR spectroscopy of FT-IR measurement. Lithium carbonate was mainly observed as the surface material and the intensity of IR absorption peaks were depended on state of charge (SOC) of the batteries. The result suggests that the amount of lithium carbonate increases by discharge and decreases by charge. (C) 2011 Elsevier B.V. All rights reserved.