The role of antisite defect pairs in surface reconstruction of layered AMO2 oxides: A DFT+U study

Abstract

Layered transition metal oxides used as cathodes in modern metal-ion batteries are prone to surface reconstruction, which is a key reason for electrochemical performance degradation. Despite extensive research on this class of materials the mechanism of surface reconstruction is still not fully clear. In this work, we use density functional theory to study the influence of antisite defect pairs on the surface structure and energetics in pristine oxide materials: LiNiO2, LiCoO2, NaNiO2, and NaCoO2. Wullf constructions were built to predict thermodynamically favorable surfaces and their orientation, which are in agreement with experiments. The energetics and structure were studied and compared for surface and bulk antisite defect pairs, which can be accompanied by transition metal charge disproportionation to +2 and +4, with the latter corresponding to the formation of a small hole polaron. Individual antisite complexes at the surface are found to be additionally stabilized opposite to those in the bulk, while no excess energy is released for the complete antisite-based surface reconstruction. These results provide insights for controlling the morphology of cathode particles and shed light on surface reconstructions in layered oxides.