Design of an Online Electrochemical Mass Spectrometry System to Study Gas Evolution from Cells with Lean and Volatile Electrolytes.

Gas evolution in high-energy Li-ion batteries remains a pervasive problem for a multitude of chemistries, jeopardizing the electrochemical performance and safety for consumers of electric vehicles. Many electrode-electrolyte degradation processes evolve gasses that may be detected in-situ with online electrochemical mass spectrometry (OEMS). In this work, details are provided for the setup and validation of an OEMS system that operates well under lean and volatile electrolyte conditions. Quite notably, the OEMS cells with only 40 µL of electrolyte and intermittent headspace sampling exhibit comparable electrochemical performance to flooded coin-cells. It is demonstrated that the onset time, shape, and magnitude of the gas evolution profiles calculated from mass spectrometer measurements match well to a known pressure reference through the use of an empirically determined fraction of removal. The off-gassing characteristics from a set of layered-oxide materials, NMC532, NMC811, and LNO, are used to further validate the OEMS setup against the literature. It is shown that many of the features present in the OEMS curves for equivalent systems from other groups are captured by this OEMS system. At an upper cut-off voltage of 4.4 V, LNO exhibits an intense release of CO 2 , O 2 , and CO gas relative to NMC532 and NMC811.