Other gases: The second most common gas observed for both electrolytes is CO2, which occurs mainly in the first cycle. The first cycle signal is also clearly different from the following ones, especially for 2G. The strong evolution in the first cycle can be attributed to the decomposition of carbonate impurities from the electrode material, as described for Li cells . It is triggered by the presence of protic species, which can be formed via oxidation at elevated potentials. CO2 evolution begins for both electrolytes during the first cycle, at the start of the high voltage plateau, suggesting that protic decomposition products are formed there. Since the chemical reaction of the carbonates consumes protic species , this could be another explanation for the H2 evolution in the first cycle also being different and less intense compared to the following ones.