Several 18650 lithium-ion batteries using LiCoO2, LiMn2O4, and LiFePO4 as cathode materials were assembled separately. Gas species of these batteries under normal cycling and overcharging to 4.5 and 5.0 V conditions were examined by means of GC-MS method. Under the normal charge and discharge voltage range, it is found that gas components are independent to the cathode materials. C2H5F gas was detected in all cases. A formation mechanism is proposed. Under overcharging condition, it is found that the gas components are different and there is a correlation between the C,H, product and the oxidation ability of various delithiated cathode materials. (c) 2004 Elsevier B.V All rights reserved.