Gassing at elevated temperature is the main reason for the performance degradation of lithium titanate (Li4Ti5O12, LTO) batteries. In this study, an in-situ device was developed and used to study on-line the transient gassing of custom-made 4.5Ah LTO/NCM pouch batteries at 1C cycling at 55 degrees C. The gas volume and internal pressure of the batteries were recorded on-line for 1000 h, and the composition of the gas components at different times was also analyzed by on-line gas chromatography. The results show that H-2 and CO2 are the main gas components. The H-2 percentage decreases, while the CO2 percentage increases gradually during the process of gassing. According to the rate-controlling step of gassing from H-2 formation reaction to CO2, a stage-by-stage mixed gassing mechanism is proposed, where the water decomposition is dominant in the initial stage and solvent decomposition is dominant in the subsequent stage. The gassing of LTO batteries aged at 55 degrees C was studied on-line at different states of charge (0%, 50%, 100% SOC). The results show that the volume and composition of the gas are essentially independent of the SOC of batteries and that formation of SEI at LTO interface is the main reason for the gassing rate reduction when aged and cycled (final phase) at 55 degrees C. (C) 2019 The Electrochemical Society.