Sustainable LiCoO ₂ by collective glide of CoO ₆ slabs upon charge/discharge

Significance

Traditional views indicate that the well-known layered LiCoO ₂ cathode delivers a typical solid-solution reaction upon delithiation. The problem is that “solid solution” is a vague concept, and the phase transition remains ambiguous. Here, we reveal a mechanism with the collective and quasi-continuous glide of CoO ₆ slabs in layered LiCoO ₂ through combining in situ XRD and ex situ STEM characterizations. Such a delithiation mechanism does not involve the nucleation-and-growth-type delithiation process and represents a completely different manner from the conventional two-phase or solid solution–phase transition processes. The lessons provide a different insight into understanding the working mechanism of layered oxide materials.

Abstract

In pursuit of high-energy-density materials, layered LiCoO ₂ has always drawn significant attention for Li-ion batteries (LIBs). Upon delithiation, LiCoO ₂ usually suffers from deleterious CoO ₆ slab glide, during which the emerging significant, but subtle, structural changes actually provide the necessary fundamentals to stabilize its high-energy-density feature, although it remains ambiguous. In this context, an unprecedented kinetic process of the CoO ₆ slab is observed upon the delithiation of LiCoO ₂. Such a behavior corresponds to a collective and quasi-continuous migration process of the CoO ₆ slabs over a wide range of charge/discharge before the layered-to-rock-salt-phase transformation. By introducing a gradual angle, the movement of CoO ₆ slabs can be precisely described with Li contents, which unlocks the door to elucidating the nanoscale electrode process dynamics as well as the atomic-scale design of layered electrodes for batteries.