Evolution of the morphology, structural and thermal stability of LiCoO2 during overcharge

Abstract

The overcharge behaviors of 1000 mAh LiCoO2/graphite pouch cells are systematically investigated through the analysis of morphology, structural and thermal stability of LiCoO2 under different state of charge (SOC) of 120%, 150%, 174%, 190% and 220%. The LiCoO2 experiences the phase transition from O3 to H1-3 and then O1 together with the grain breakage and boundary slip as evidenced by XRD and SEM analysis respectively, which results in the pronounced Co dissolution after the SOC of 174%. The impedance analysis of the pouch cells and coin cells demonstrates that the main contribution of impedance increase originates from the LiCoO2 electrode. Under the combined effect of structural collapse, Co dissolution, and electrolyte oxidization, the thermal stability of LiCoO2 cathode materials reduces with the progressing of overcharge as revealed by differential scanning calorimetry (DSC) results. We hope that this comprehensive understanding can provide meaningful guidance for advancing the overcharge performance of LiCoO2/graphite pouch cells.