様式３S22021

Reversibility of Anionic Redox Reaction in Lithium-rich Co-based Positive
Electrodes with Cation Disordered Rocksalt Structure
Zhang Yanjia a, Naoaki Yabuuchia, Daisuke Shibata b, and Toshiaki Ohta ba

Department of Chemistry and Life Science, Yokohama National University, bThe SR Center, Ritsumeikan
Universitye-mail: yabuuchi-naoaki-pw@ynu.ac.jp

Li-excess electrode materials potentially boost the energy density of Li-ion batteries, but the origin ofactivation of anionic redox in these electrode materials is still under debate. To study the anionic redox
reversibility in Li3NbO4–CoO binary system, Co L-edge and O K-edge XAS spectra are collected for Li7/6–yCo 4/9Nb7/18O2 at different charge states. XAS study reveals that partially reversible anionic redox isachieved for Li7/6–yCo4/9Nb7/18O2, which is theoretically predicted by the formation of linear Co-O-Co
environment for a cation disordered system.
Keywords: lithium-ion batteries, positive electrode material, anionic redox, Co L-edge spectra, O K-edge
spectra
背景と研究目的 (Introduction)
In recent years, towards fossil fuel-free society, a shift from internal combustion engine vehicles toelectric vehicles is accelerating. To further increase its speed, it is necessary to develop high-energy
density positive electrode materials. Research on positive electrode materials using anionic redox withhigher energy density, when compared with classical layered materials with cationic redox, is expanding inrecent years. However, many materials with anionic redox are composed of Mn ions, and therefore theknowledge of anionic redox with other transition metal ions is limited. Most recently, soft XAS study at
the SR center reveals that anionic redox is activated in materials with Ni ions, which originates fromstructural disordering coupled with the formation of linear Ni-O-Ni environment[1]. Therefore, in thisstudy, to deepen our understanding on anionic redox with different transition metal ions with a disorderedrocksalt structure, soft XAS study is conducted with electrode materials with Co ions. For this purpose,
Li7/6Co4/9Nb7/18O2 with a disordered rocksalt structure is synthesized using a solid-state calcination
reaction from precursors containing Li, Co, and Nb ions. For comparison of charge compensation
mechanism and reversibility of anionic redox, lithium-rich system, Li4/3Co2/9Nb4/9O2 was also synthesized,and both samples, Li7/6Co4/9Nb4/9O2 and Li4/3Co2/9Nb4/9O2, are found for solid solution oxides in Li3NbO4–
CoO binary system.
実験 (Experimental)
Li7/6Co4/9Nb4/9O2 was synthesized by solid-state calcination. Li 2CO3 (98.5%, Kanto Kagaku),
CoCO3•Co(OH)2•4H2O (Wako Pure Chemical Industries), and Nb 2O5 (99.9%; Wako Pure ChemicalIndustries) were mixed by wet ball-milling at 300 rpm for 6 h with ethanol. The obtained powder was thenpressed into a pellet under a pressure of 20 MPa. The pellet was transferred into a quartz tube and calcinedin Ar at 1000 °C for 2 h. The electrode performance of samples was tested using two-electrode cells (TJAC, Tomcell Japan). The composite positive electrode consisted of 80 wt% active material, 10 wt%
acetylene black (HS-100, Denka) and 10 wt% poly(vinylidene fluoride) (#1100, Kureha), pasted on analuminum foil, then the slurry was dried in vacuum, and before assembling cells a positive electrode wasfurther dried at 120 °C for 2 h in vacuum. Metallic lithium (Honjo Metal) was used as a negative electrode.
The electrolyte solution used was 1.0 mol dm –3 LiPF6 dissolved in ethylene carbonate:dimethyl carbonate(3:7 by volume) (battery grade, Kishida Chemical). A polyolefin microporous membrane was used asseparator. A constant current charge-discharge test was conducted. Regarding the change in the charge
compensation mechanism, Co L-edge and the O K-edge XAS spectra of the samples were collected at theSR Center, BL-11, in Ritsumeikan University.

様式３
結果、および、考察 (Results and discussion)：
To study the charge compensation mechanisms in Li 3NbO4–CoO binary system, Co L-edge and O Kedge XAS spectra of Li7/6Co4/9Nb7/18O2 were measured at different charge states, and results are
summarized in Fig. 1. For Co L-edge XAS spectra, the as-prepared Li7/6Co4/9Nb7/18O2 contains Co 2+ with ahigh-spin d7 configuration (up-spin t2g3 eg2, down-spin t2g2), similar to CoO with a disordered rocksalt
structure[2]. After charge to 80 mA h g–1, the oxidation state of Co is increased to the trivalent state as
noticed from the energy shift of Co L-edge XAS spectra to a higher energy region. However, there is still ashoulder peak at lower energy, indicating divalent Co ions also remain. Upon further charge to 160 mA h
g–1, Co was oxidized to trivalent state, the shoulder peak corresponding to divalent state disappeared.
There is no change of Co L-edge XAS spectra after fully charge.
Because the covalency of Co is increased, the energies of O K-edge XAS spectra are also graduallyshifted to a lower energy region (from pristine to 160 mA h g–1). Noticeably, a small but clear peak
appears at O K-edge XAS spectra at 527 eV upon charge, which is not observed for LiCoO 2 and LaCoO3
with trivalent Co ions. Moreover, this peak is also observed for SrCoO3 synthesized by high-pressure
method, and this peak for O K-edge XAS spectra is assigned to a ligand hole[2]. This fact suggests thatligand holes are partially formed in Li7/6–yCo4/9Nb7/18O2. Our DFT study also supports that this trend andligand holes are formed in oxygen, which have a linear Co-O-Co environment. This environment is onlyformed in the disordered rocksalt structure and not formed in layered structure. Moreover, such ligand holecannot be stabilized in lithium-rich system and oxygen loss is evidenced for Li 4/3–yCo2/9Nb4/9O2 because Li
enrichment results in the loss of linear Co-O-Co environment for oxygen.
These findings in this study, potentially contribute to a better understanding of anionic redox
reversibility in Co-based oxides, leading to further development of high-energy electrode materials withanionic redox in the future.

Co L-edge
PFYCo3+Co2+O K-edge
PFY2+Li7/6CONbCo4/94/9
NbOO2 2
7/18
7/18

pristine
80 mAh g-1
160 mAh g-1
full charged

ligand holepristine
80 mAh g-1
160 mAh g-1
full charged

770

775

780

785

790

795

Energy / eVFig. 1.

800

805

810

520

525

530

535

540

545

Energy / eV(left) Co L-edge and (right) O K-edge XAS spectra of Li7/6–yCo4/9Nb7/18O2 with different charged states.

参考文献[1] R. Fukuma et al., and T. Ohta, N. Yabuuchi, ACS Cent. Sci., 8, 775 (2022).
[2] Y. Zhang et al., and D. Shibata. T. Ohta, N. Yabuuchi, J. Phys. Chem. C, 127, 2194 (2023).
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研究成果公開方法／産業への応用・展開について
・This study has been published in JPCC as shown in reference [2].