Evaluation of Electronic and Crystal Structures of Li-Rich Layered Oxide Cathode Using Combinations of Exes, XPS, and PDF Analyses
Authors: Oishi Masatsugu , Kawaguchi Tomoya , Hamamoto Raku , Hiroi Satoshi , Ohara Koji , Okamoto Norihiko , Ichitsubo Tetsu 
Published: December 2024

Lithium-rich layered oxides (LLOs) have attracted attention as high-capacity cathode materials for lithium-ion secondary batteries. Its unique charge-compensation mechanism and structure changes during delithiation and lithiation yield a large capacity. LLOs reversibly extract and insert ~1 mol of Li (> 250 mAh/g) per Li1+x

Me
1
−

x
O2 (Me=Ni, Co, Mn) notation. In contrast, conventional layered oxides such as LiCoO2 reversibly extract and insert ~0.6 mol (~160 mAh/g), which is limited by the irreversible change of the host structure by the excessive Li extraction. However, such a large amount of delithiation in LLOs still induces various structural changes. Although various structure analyses, including conventional XRD, have revealed the structural changes of LLOs accompanying charging and discharging, the inherently disordered structure of LLOs requires a more elaborate experimental approach to elucidate the relationship between the structural changes and electrode properties. Because conventional XRD only provides structure information of the well-ordered crystalline phase, the structure evolution of LLOs and their charging/discharging mechanisms should be elucidated to reveal such disordered structures. Pair distribution function (PDF) analysis, which is an advanced XRD technique using X-ray total scattering, enables quantitative observations of the short-, medium-, and long-range orders with high spatial resolutions in real space even from highly disordered structures simultaneously. In addition, spectroscopic analyses have revealed the unique charge compensation mechanisms associated with the high capacity of LLOs. In LLOs, the contribution of O anions is a key factor for the reversible high capacity.

Another consideration is the valence bands of both O and Me from simultaneous observations of constituent elements, since the O 2p and Me 3d orbitals are hybridized. Soft X-ray emission spectroscopy (SXES) is a useful technique to observe the electronic structure of multiple elements simultaneously. Among them, electron-induced X-ray emission spectroscopy (EXES) is an accessible technique using laboratory X-ray or electron beam sources for excitation. In this study, we performed EXES and PDF analyses along with X-ray photoelectron spectroscopy (XPS) on the same states of samples for the LLO cathode, 0.4Li2MnO3−0.6LiNi0.5Mn0.5O2, to reveal the relation between the charge compensation mechanism and changes in the crystal structure. The PDF analysis suggested that charging caused Me cations to migrate to the octahedral and tetrahedral sites in the Li layer. EXES independently observes the energy distribution of each orbital, revealing their specific roles in the charge compensation accompanying electrochemical charging and discharging. EXES with the aid of XPS revealed that the charge was compensated by Ni, Mn, and O. The contribution from each element depended on the charging/discharging state. The changes in the electronic/crystalline structures that yield the unique charge-compensation mechanism and the large capacity of the LLO electrode will be discussed.