Atomic-scale insight into the lattice volume plunge of LixCoO2 upon deep delithiation

Abstract

The practical capacity utilization of LiCoO₂ is limited to 50–70% due to the dramatic volume shrinkage induced cracks and subsequent interface parasitic reactions at high voltage. However, the fundamental understanding of the dramatic lattice volume shrinkage remains unclear. In this work, we discover that the delithiation paths have an impact on the lattice volume turning point of Li_xCoO₂, where the corresponding capacity utilization can be as low as 62.5% or as high as 75% consequently. In addition, the O ↔ O interlayer distance across the Li layer (O_(d3)) mainly contributes to the volume increase before 62.5–75% delithiation, and the O ↔ O interlayer distance across the Co layer (O_(d2)) is the dominant factor for the dramatic volume decrease after the volume turning point. The electron localization function (ELF) around O keeps increasing during delithiation and it increases significantly after more than 62.5% delithiation, indicating that the lattice oxygen participates in charge compensation during the whole delithiation process and it becomes the main contributor to the charge compensation at a high delithiated state compared with Co. This work unravels the fundamental reason for the dramatic volume shrinkage of Li_xCoO₂ at high voltage. It claims that the antisite defects (Co_Li) of LiCoO₂ should be designed carefully owing to the weaker Co–O bond strength and further oxygen release, which accelerate the degradation of LiCoO₂, although it can increase the voltage of LiCoO₂. More importantly, LiCoO₂ material design with elemental doping, oxygen defects (V_O), Li defects (V_Li), and Co defects (V_Co) will contribute to the capacity utilization of LiCoO₂.