As hazardous situations can occur during the life of a Li-ion battery, it is of great importance to understand its behavior under abusive conditions (mechanical, thermal or electrical). In particular, the study of overcharge, which consists of forcing a current through the cell, can be very helpful in improving battery safety. Very few studies in the literature have focused on the chemical reaction mechanism responsible for failure during overcharge. This is, however, of great interest because a Li-ion battery can produce reactions in a sealed container and is thus a highly reactive system. Here, experimental approaches are employed to understand the reaction mechanisms that occur during overcharge testing. Experiments consist of studying the overcharge kinetics of a commercial battery at an initial state of charge of 100%. The battery is maintained in a known volume and gaseous samples are withdrawn both at the end of the test and continuously during the test. The main gaseous species are then identified and quantified by gas phase chromatography coupled with mass spectrometry and FTIR spectroscopy. This experimental study is completed by a numerical investigation to determine the combustion parameters of the exhaust gases using a detailed reaction mechanism associated with a numerical code.