Nonrigid Band Behavior of the Electronic Structure of LiCoO₂ Thin Film during Electrochemical Li Deintercalation

Abstract

In this study, a comprehensive experimental in situ analysis of the evolution of the occupied and unoccupied density of states as a function of the charging state of the Lix≤1CoO2 films has been done by using synchrotron X-ray photoelectron spectroscopy (SXPS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and O K- and Co L3,2-edges XANES. Our experimental data demonstrate the change of the Fermi level position and the Co3d–O2p hybridization under the Li+ removal and provide the evidence for the involvement of the oxygen states in the charge compensation. Thus, the rigid band model fails to describe the observed changes of the electronic structure. The Co site is involved in a Co3+ → Co4+ oxidation at the period of the Li deintercalation (x ∼ 0.5), while the electronic configuration at the oxygen site is stable up to 4.2 V. Further lowering of the Fermi level promoted by Li+ extraction leads to a deviation of the electronic density of states due to structural distortions, and the top of the O2p bands overlaps the Co3d state which is accompanied by a hole transfer to the O2p states. The intrinsic voltage limit of LiCoO2 has been determined, and the energy band diagram of Lix≤1CoO2 vs the evolution of the Fermi level has been built. It was concluded that LixCoO2 cannot be stabilized at the deep Li deintercalation even with chemically compatible solid electrolytes.