In situ analysis and mechanistic understanding of gas species generated in battery cells during thermal abuse

Understanding gas generation in lithium-ion batteries during thermal runaway is critical to designing safer electric vehicles. We developed an in situ gas analysis system capable of measuring gases as they are generated inside a lab-scale battery cell during thermal abuse. Two phases of gas-generating reactions were observed in charged Lithium Nickel Cobalt Manganese Aluminum Oxide (NCMA)-graphite cells. By adding a lithium iron phosphate (LFP)-based reference electrode inside the cell, we find that reactions occurring between the anode and electrolyte generate H2 and CO2 in the 80–130 °C temperature range. These reactions are correlated with the self-heating onset observed in accelerated rate calorimetry (ARC) and involve both solid electrolyte interphase (SEI) and intercalated lithium. Above 160 °C, reactions occurring due to cathode decomposition accelerate thermal runaway and generate large amounts of carbon dioxide, and to a lesser extent, hydrogen and ethylene. The methods presented herein can be used to evaluate cell thermal stability for the design of safer batteries.