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

Daisuke Asakura*, Takaaki Sudayama, Yusuke Nanba, Eiji Hosono, Hisao Kiuchi, Kosuke Yamazoe, Jun Miyawaki, Yoshihisa Harada, Atsuo Yamada, Ru Pan Wang, Frank M.F. de Groot

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

Abstract

To understand the electronic-structure change of LiCoO2, a widely used cathode material in Li-ion batteries, during charge-discharge, we performed ex situ 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 vs. 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,.

Original languageEnglish
JournalPhysical Chemistry Chemical Physics
DOIs
Publication statusE-pub ahead of print - 10 Dec 2024

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