Optimal facet assembly of multi-transition metal layered cathodes toward superior Li-ion kinetics and structural stability

Abstract

The surface stability of layered cathode materials is a critical determinant of their chemo-mechanical reversibility during intercalation/deintercalation. This is possibly because of the migration of transition metals (TMs) into the Li layer followed by O redox, which decreases the O vacancy formation energy and thereby destabilizes the anionic framework. Hence, elucidating an optimal assembly of surface facets is a key strategy for further stabilizing the characteristic R3m structure to allow higher capacities and states of charging. In this study, three single-crystal samples of LiNi_0.33Co_0.33Mn_0.33O₂ (NCM111) with varied surface facets were synthesized and compared. After cycling, (012) facets had higher losses of Li and O atoms than (104) facets, leading to excessive Mn reduction. The formation of rocksalt structures and the loss of Li-ion migration pathways during extended cycling primarily resulted from irreversible compositional change of the (012) facets. Through electrochemical single-particle simulations we could establish a correlation between degraded surface structures and inferior electrochemical properties. The simulation indicated that the complete degradation of (012) facets is accompanied by an overpotential of approximately 0.2 V, leading to severe voltage hysteresis, which agrees well with our experimental findings. This study provides an important insight into the ideal assembly of surface facets in layered cathode materials that will help future cathode research realize more stable surface structures with even higher capacities and states of charging.