The Impact of the C-Rate on Gassing During Formation of NMC622 II Graphite Lithium-Ion Battery Cells

Formation is considered a cost and time intensive production step in industrial production of lithium-ion batteries (LIBs). One solution for improvement is an acceleration of the formation step by applying higher C-rates. In this study, formation protocols with up to 2 C were applied to LiNi0.6Mn0.2Co0.2O2 (NMC622) II graphite pouch cells with a nominal capacity of 5 Ah. The formation protocols utilizing higher C-rates result in a decrease in overall formation time, but also in increased gassing due to additional electrolyte decomposition. The resulting gas phase was quantitatively determined using gas chromatography-barrier discharge ionization detection (GC-BID) and with regard to gas volume by making use of the Archimedes principle. Main formation gases in these cells were identified as CO and C2H4. Increased C-rates altered certain decomposition reactions. Especially the CO evolution was increased. Nevertheless, gassing was smallest for 0.1 C and increased with lower and higher C-rate. In case of too low C-rates hydrogen was identified as the main formation gas. However, higher gassing was not correlated with higher capacity loss during formation. Furthermore, the dependence of the C-rate on gassing was found to be dependent on the graphite material.