A new insight into continuous performance decay mechanism of Ni-rich layered oxide cathode for high energy lithium ion batteries

Ni-rich layered lithium transition metal oxides (LTMO) are regarded as one of the most potential candidates to usher in a new stage of the ultra-high available energy density lithium-ion batteries (LIBs). However, the severe capacity and voltage fading remain a big challenge on the practical application, while lacking of atomic scale evidence makes the performance degradation mechanism of Ni-rich LTMO essentially ambiguous. Here we report a more accurate study on the detailed structural transformation and chemical evolution processes upon cycling to shed light on the performance decay from the perspective of variation on the nature of the stoichiometric Ni-rich LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode. A novel continuous structural evolution mechanism at atomic scale based on the migration of transition metal cations into lithium ion diffusion channels has been proposed to acquire a new insight into the energy decay behaviour of Ni-rich cathode. It is demonstrated that Ni would migrate from bulk to surface along with the irreversible reduction by virtue of the low diffusion barrier and the Ni concerntration gradient in lattice, resulting in the growth of structural restruction layer (SRL) throughout the whole charge/discharge processes and the ongoing performance decay. Thus, the future works on achieving higher available energy and longer cycle life for Ni-rich layered cathodes should focus on how to prevent the migration of transition metal ions.