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 5.0V 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 C2H2 product and the oxidation ability of various delithiated cathode materials.