High accuracy in-situ direct gas analysis of Li-ion batteries

Cycling of lithium-ion batteries containing Ni-rich NMC cathodes at high voltage involves intense gas generation. From a safety standpoint, it is critical to understand how different gas species respond to changes in upper cut-off voltages. In this manuscript, we introduce a novel experimental set up for real-time analysis of gas generation in prismatic pouch cells. In a typical experiment, a lithium-ion pouch cell is directly connected to a quadruple mass spectrometer using a glass capillary. Pressure difference helps move the generated gases to the mass spectrometer column for full analysis. The gaseous species are probed during both formation cycle and aging cycles in Li-ion pouch cells comprising NMC811 cathodes and graphite anodes. The gases are generated upon the creation of the solid electrolyte interphase on graphite during the first formation charge. Ethylene (C2H4) is the major gas generated during formation cycle due to the decomposition of ethylene carbonate. H2, CH4, C2H6, O2, CO and CO2 are also monitored during formation cycle. For the aging cycles, three upper cut-off voltages 4.2, 4.4 and 4.6 V are selected to investigate the impact of upper cut-off voltages on gas generation. Higher upper cut-off voltages do not significantly affect the amount of O2 measured, as a part of the generated oxygen could have reacted with electrolytes. On the other hand, the generation of CO2 is found to be very sensitive to upper cut-off voltage. During similar aging cycles, 6167 nmol gNMC−1 of CO2 is generated in 4.6 V cycle, versus 1650 nmol gNMC−1 in 4.4 V cycle, versus 91 nmol gNMC−1 in 4.2 V cycle.