【公開日：2025.06.10】【最終更新日：2025.04.22】

課題データ / Project Data

課題番号 / Project Issue Number

24QS0109

利用課題名 / Title

Redox and structural behavior study of cathode material via RIXS and HERFD-XAS techniques

利用した実施機関 / Support Institute

量子科学技術研究開発機構 / QST

機関外・機関内の利用 / External or Internal Use

外部利用/External Use

技術領域 / Technology Area

【横断技術領域 / Cross-Technology Area】（主 / Main）計測・分析/Advanced Characterization（副 / Sub）-

【重要技術領域 / Important Technology Area】（主 / Main）革新的なエネルギー変換を可能とするマテリアル/Materials enabling innovative energy conversion（副 / Sub）量子・電子制御により革新的な機能を発現するマテリアル/Materials using quantum and electronic control to perform innovative functions

キーワード / Keywords

materials for power generation, x-ray absorption near edge structural analysis, x-ray fluorescence spectroscopy, Li-ion battery, electrode, x-ray absorption spectroscoy, x-ray emission spectrosocpy,放射光/ Synchrotron radiation,電極材料/ Electrode material,表面・界面・粒界制御/ Surface/interface/grain boundary control,二次電池/ Secondary battery

利用者と利用形態 / User and Support Type

利用者名（課題申請者）/ User Name (Project Applicant)

Zhang Jing

所属名 / Affiliation

Institute of High Energy Physics, Chinese Academy of Sciences

共同利用者氏名 / Names of Collaborators in Other Institutes Than Hub and Spoke Institutes

Chu Shengqi,Xia Yinghao,Jia Xun,Li Zelong

ARIM実施機関支援担当者 / Names of Collaborators in The Hub and Spoke Institutes

石井 賢司

利用形態 / Support Type

（主 / Main）共同研究/Joint Research（副 / Sub）-

利用した主な設備 / Equipment Used in This Project

QS-112：共鳴非弾性X線散乱装置

報告書データ / Report

概要（目的・用途・実施内容）/ Abstract (Aim, Use Applications and Contents)

Aim—Suppression of voltage decay of lithium-rich materials by regulating spin state of Mn³⁺ Layered lithium-rich manganese-based oxide (LLRO) materials are the most promising next-generation cathode materials for lithium-ion batteries due to their simultaneous cationic and anionic redox. However, the major issue limiting their commercialization is the gradual decrease in average discharge voltage during cycling, which not only reduces energy density but also complicates battery management. Among the explanations related to the origin of voltage decay, it has reached a consensus that the interlayer migration of Mn ions during electrochemical cycles, leading to changes in the local Mn coordination environment and, consequently, the increased reduction potential (v.s. Li⁺/Li). Due to factors of oxygen loss and minimizing the energy, Mn⁴⁺ inevitably obtained electron and reduction to Mn³⁺ during cycling. Mn³⁺ can exist in two different electronic configurations due to changes in coordination environment, spin-orbit coupling effect and changes in 3d orbital splitting energy caused by electron interactions: 1) High-spin Mn³⁺, with the electron configuration of t_2g³ e_g¹; 2) Low-spin Mn³⁺, with the empty e_g state occupation of t_2g⁴ e_g⁰. High-spin Mn³⁺ is a Jahn-Teller active ions, which leads to octahedral distortion and in turn induces structural stress. In addition, high-spin ions easily migrate due to a more dispersed electron cloud, which interacts more weakly with the ions in the lattice. In contrast, low-spin ions have more localized electron density and stronger interaction with surrounding ions, leading to higher migration barriers. Therefore, as shown in Fig. 1, the spin state regulation of Mn³⁺ through short-range coordination structure around Mn ions are the effective ways to suppress detrimental Mn ion migration, finally reducing voltage decay. According to the principle of TM-O-TM superexchange, we propose that a small amount of Co substitution can cause Mn³⁺ to transform into a low-spin state. Specifically, when Co³⁺ is introduced, there is an atom arrangement of Co³⁺-O^2--Mn³⁺, and the 2p orbital of O overlaps with the connected TM 3d orbital. At the deep delithiated state, Co⁴⁺(t_2g⁵ e_g⁰) is reduced to spin-free Co³⁺(t_2g⁶ e_g⁰) through the ligand-metal charge transfer effect (LMCT). At this time, the O 2p orbital adopts a 2p⁵ electronic configuration with a net spin-up polarization (assuming that O loses a spin-down electron). This induces an antiparallel spin alignment between the O 2p and Mn³⁺ 3d orbitals via exchange interactions, further stabilizing the Mn³⁺ in a low-spin state due to crystal field splitting.

Use Application and Contents—Analysis of Mn³⁺ spin states and short-range structures using HERFD XAS and XES Due to the limited broadening of the core-hole lifetime in ordinary X-rayabsorption spectra, the signal of the Mn element pre-edge peak cannot be well resolved, and a lot of information including the spin state will be ignored. Therefore, it is necessary to use high-energy-resolution synchrotron radiation XAS or XES to explore this experiment. This proposal aims to clarify: (a) the role of TM-O-TM superexchange interactions in the regulation of Mn³⁺ spin states; (b) the correlation between Mn³⁺ spin state and Mn migration. The synchrotron radiation high-energy resolution fluorescence detection X-ray absorption and emission spectroscopy were conducted out (HERFD XAS and XES) to address these two issues. Specifically, to reveal the former, we will analyze the splitting energy of the Kβ' and Kβ_1,3 peaks in the Mn Kβ fluorescence XES, which are caused by spin-orbit coupling, to investigate magnetic ordering evolution. Additionally, by combining Mn XAS, we will deconvolute the pre-edge region to track the electron transitions between 1s to 3d orbital before and after 100 cycles, revealing the orbital occupancy. To address the later, we will apply Fourier transform and wavelet transform on EXAFS spectra, and through fitting, investigate the changes in the local Mn environment, including the evolution of the first shell coordination number and disordered factors. Recently, we successfully constructed low-spin Mn³⁺ via superexchange interactions (validated through XAS and XES, as shown in the Fig. 2), and confirmed via EXAFS that there was a minimal change in the Mn short-range structure.

実験 / Experimental

Two materials, conventional layered lithium-rich oxide without Co elements and cathode with Co elements substitution, were prepared. Two states with uncycled electrode and cycled electrode after 100 times were selected for both materials, respectively. Mn K-edge HERFD XAS and XES  were performed on them, respectively.

結果と考察 / Results and Discussion

Results and Discussion 
1.  Through a targeted Co-substitution strategy coupled with compatible synchrotron characterization (e.g., HERFD XAS/XES and XANES/EXAFS), we decoupled the Mn³⁺ spin state evolution from competing structural factors, enabling direct correlation between Co-induced superexchange interactions and dynamic Mn/O coordination symmetry during delithiation.
2.  Quantitative analysis of Mn Kβ_1,3-Kβ' splitting (XES) and pre-edge absorption features (XAS) confirmed Co-driven stabilization of low-spin Mn³⁺ (t_2g⁴e_g⁰), while electrochemical measurement revealed a 169 mV after 100 cycles, compared with that of 350 mV from the material without Co substitution(shown in Fig. 3).
3.  The research on voltage decay suppression of lithium-rich cathode materials relies on the redox behavior of TM ions and changes in short-range structures.
In the future, we hope to determine the d orbital energy levels, valence band and conduction band distribution of TM ions, and their interactions through RIXS to better reveal the evolution of TM ions in electrochemical cycles.

図・表・数式 / Figures, Tables and Equations

Fig. 1  Schematic diagram of the mechanism of alleviating voltage decay by regulating the spin state of Mn³⁺

Fig. 2 Mn K-edge HERFD XAS and XES of two different spin states

Fig. 3 Voltage decay performance of materials with two different spin states

その他・特記事項（参考文献・謝辞等） / Remarks(References and Acknowledgements)

1. Li N, Hwang S, Sun M, et al., Unraveling the Voltage Decay Phenomenon in Li-Rich Layered Oxide Cathode of No Oxygen Activity, Advanced Energy Materials, 2019, 9(47).
2. Li N, Sallis S, Papp J K, et al., Correlating the phase evolution and anionic redox in Co-free Ni-rich layered oxide cathodes, Nano Energy, 2020, 78: 105365.
3. Li Y, Zhu X, Su Y, et al., Enabling High-Performance Layered Li-Rich Oxide Cathodes by Regulating the Formation of Integrated Cation-Disordered Domains, Small, 2024: 2307292.
4. Li N, Sallis S, Papp J K, et al., Unraveling the Cationic and Anionic Redox Reactions in Conventional Layered Oxide Cathode, ACS Energy Letters, 2019, 4(12):2836-2842.

成果発表・成果利用 / Publication and Patents

論文・プロシーディング（DOIのあるもの） / DOI (Publication and Proceedings)

口頭発表、ポスター発表および、その他の論文 / Oral Presentations etc.

特許 / Patents

特許出願件数 / Number of Patent Applications：0件
特許登録件数 / Number of Registered Patents：0件