Improved volumetric energy density of the battery system is requested to increase the range of upcoming electric cars. To guarantee safety of enhanced Li-ion batteries in electric vehicles operation as well as for transport and storage is of crucial importance for acceptability of this technology. Failure cases of Li-ion batteries causing high temperatures and undesirable chemical reactions which can result in exothermic reactions and furthermore thermal runaway have to be avoided reliably. This paper presents potential failure cases of large batteries and approaches how to detect these failures. Important parameters as well as the composition of gas release with the focus on hydrogen fluoride (HF) are provided. The aim is to improve gas composition measurement, to learn more about the Li-ion failure cases and to use early failure detection for preventing the release of harmful toxic and inflammable gases. In this paper the gas composition of the failure case “second degassing” is measured and analyzed with an FTIR spectrometer.