Origin of the different degradation mechanisms of LNCM and LNCA cathodes in Li-ion batteries

Abstract

Lithium nickel cobalt manganese oxide (LNCM) and lithium nickel cobalt aluminum oxide (LNCA) display similar performances and characteristics as cathode materials, but their degradation behaviors differ. To investigate the origin of these differences, the properties of LNCM and LNCA are comparatively examined computationally. Their structural, electronic, and transport properties show no significant differences, indicating that the degradation mechanisms cannot be explained through these intrinsic properties. Phase equilibria simulation shows that Mn embedment in the crystal is thermodynamically and kinetically favored; thus, the Mn concentration should be homogeneous over the LNCM particles. However, the Al distribution varies based on synthetic conditions, which can cause uneven concentration distributions or secondary-phase formation. In addition, the LNCA volume change with variations in Al concentration is more severe than that of LNCM with Mn concentration. Thus, LNCA particles may experience higher internal mechanical stresses, whereas the surfaces are protected by the secondary-phase coating effect. These features give LNCA robust surfaces but vulnerability to internal stress-induced particle breakage, while LNCM has relatively stable bulk properties but suffers surface-related degradation owing to bare surface exposure. This interpretation agrees well with the reported characteristic degradation behaviors of LNCM and LNCA, thus properly explaining the underlying mechanisms.