Abstract
Cycling of lithium-ion batteries containing Ni-rich NMC cathodes at high voltage involves intense gas generation. From the safety standpoint, it is critical to understand how different gas species respond to changes of 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 by 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 cathode and graphite anode. The gases are generated upon the creation of the solid electrolyte interphase on graphite during the first formation charge. Besides ethylene gas, C2H4, whose occurrence is related to the decomposition of ethylene carbonate, a minor solvent decomposition happened at a lower voltage is demonstrated to be associated with residue water on the electrode. For aging cycles, three upper cut-off voltages have been selected to investigate the gas evolution behavior upon charging to 4.2, 4.4 and 4.6 V. Higher upper cut-off voltages do not affect the amount of O2 measured as a part of the generated oxygen could have reacted with electrolytes. Quite similar and low amounts of O2 remain in the cells upon charging to the selected cut-off voltages. On the other hand, the generation of CO2 has been found to be very sensitive to upper cut-off voltage. During similar aging cycles, 6167 nmol g_{NMC}^{-1} of CO2 is generated at 4.6 V, versus 1650 nmol g_{NMC}^{-1} of CO2 at 4.4 V, versus 91 nmol g_{NMC}^{-1} of CO2 at 4.2 V.
