Origin of Carbon Dioxide Evolved during Cycling of Nickel-Rich Layered NCM Cathodes.

Gas formation caused by parasitic side reactions is one of the fundamental concerns in state-of-the-art lithium-ion batteries because gas bubbles might block local parts of the electrode surface, hindering lithium transport and leading to inhomogeneous current distributions. Here, we elucidate on the origin of CO, which is the dominant gaseous species associated with the layered lithium nickel cobalt manganese oxide (NCM) cathode, by implementing isotope labeling and electrolyte substitution in differential electrochemical mass spectrometry-differential electrochemical infrared spectroscopy measurements. LiCO on the NCM surface was successfully labeled with C via a process that involves its removal followed by intentional growth. In situ gas analytics on such NCM samples with C-labeled LiCO clearly indicate that LiCO decomposition contributes to CO evolution, especially during the first charge. At the same time, the greater contribution of electrolyte decomposition was indicated by the large amount of CO observed. Employment of butyronitrile as the electrolyte solvent in further measurements helped determine that the majority of electrolyte decomposition occurs via a reaction that involves the lattice oxygen of NCM.