The Atomic and Electronic Structure Changes Upon Delithiation of LiCoO2: From First Principles Calculations

The physical origin of the degradation on the cycling performance of LiCoO2 upon deep lithium extraction is studied from first principles calculations. Results show that the structural stability is strongly associated with the electronic structures of Co-3d, which is very flexible and can be exhibited as different electronic configurations. In LiCoO2, Co3 + is non-magnetic and holds the (t2g ↑)3(t2g ↓)3 electronic configuration. Upon lithium deintercalation, some Co3 + ions loss one electron and become Co4 + with (t2g ↑)3(t2g ↓)2 configuration. Both structures are stable since the distortion of the CoO6 octahedral is small, and thus these structures do not contribute much to the instability. On the other hand, oxygen vacancy is one important reason to the structural instability. We found that spin flip occurs to the electronic structure of Co-3d close to oxygen vacancies. Co3 + holds the (↑)4(↓)2 electronic configuration and magnetized with 2 μB magnetic moment. Furthermore, some Co3 + obtains one electron and becomes Co2 + near the oxygen vacancy. In these cases, the charge distribution around the Co atom is not symmetric and the local structure is distorted obviously, which can further accelerate the process of the structural degradation.