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 d⁶ state ( 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.

中文翻译：

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通过在 Co L3 边缘的共振软 X 射线发射光谱阐明带电 LiCoO2 中具有强电荷转移效应的 Co4+ 状态


为了了解锂离子电池中广泛使用的正极材料 LiCoO₂ 的电子结构变化，在充放电过程中，我们进行了 Co L₃ 边缘的非原位软 X 射线吸收光谱 （XAS） 和谐振软 X 射线发射光谱 （RXES） 并结合电荷转移倍数计算。4.2 V 时带电状态的 RXES 曲线与 Li/Li⁺ 发生显著变化，对应于初始状态下 Co³⁺ 低自旋态的氧化反应，而 XAS 曲线则表现出微小的变化。对于 4.2 V 充电状态，我们确认大约一半的初始 Co³⁺ 离子被氧化为 Co⁴⁺ 离子。RXES 结果的多重计算表明，Co⁴⁺ 状态具有负电荷转移能，而 d⁶ 状态（ 是一个配体空穴）是最稳定的。因此，强电荷转移效应产生的 O 2p 空穴在 LiCoO₂ 的氧化还原反应中起主要作用。