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 CO₂, 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. Li₂CO₃ on the NCM surface was successfully labeled with ¹³C via a process that involves its removal followed by intentional growth. In situ gas analytics on such NCM samples with ¹³C-labeled Li₂CO₃ clearly indicate that Li₂CO₃ decomposition contributes to CO₂ evolution, especially during the first charge. At the same time, the greater contribution of electrolyte decomposition was indicated by the large amount of ¹²CO₂ 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.