Elucidation of the Co⁴⁺ state with strong charge-transfer effects in charged LiCoO₂ by resonant soft X-ray emission spectroscopy at the Co L₃ edge

To understand the electronic-structure change of LiCoO₂, a widely used cathode material in Li-ion batteries, during charge-discharge, we performed <i>ex situ</i> soft X-ray absorption spectroscopy (XAS) and resonant soft X-ray emission spectroscopy (RXES) of the Co L₃ edge in combination with charge-transfer multiplet calculations. The RXES profile significantly changed for the charged state at 4.2 V <i>vs.</i> Li/Li⁺, corresponding to the oxidation reaction from a Co³⁺ low-spin state for the initial state, while the XAS profile exhibited small changes. For the 4.2-V charged state, we confirmed that approximately half of the initial Co³⁺ ions were oxidized to Co⁴⁺ ions. The multiplet calculation of the RXES results revealed that the Co⁴⁺ state has a negative charge-transfer energy and the d⁶L̲ state (L̲ is a ligand hole) is the most stable. Therefore, the O 2p hole created by the strong charge-transfer effect plays a major role in the redox reaction of LiCoO₂.