Crystalline Grain Interior Configuration and Lithium Migration Kinetics in Li-Rich Layered Oxide
Authors: Yu Haijun , So Yeong-Gi , Kuwabara Akihide , Zhou Haoshen , Ikuhara Yuichi , Kudo Tetsuichi 
Published: February 2020

The lithium-rich layered oxide materials xLi2MnO3·(1-x)LiTMO2 (LLOs) are considered as the most promising cathode materials for next-generation LIBs and have been developing energetically recently.1-3However, there are many issues still unclear (the structure and reaction mechanism are ambiguous until now), and numerous scientific challenges (low initial coulombic efficiency, poor rate capability and voltage degradation during cycling) of these materials that must be overcome to realize their utilization in commercial lithium-ion batteries.

The electrochemical performance of electrode materials is largely controlled by the pathways of charge carriers in and out of a crystal and their kinetic barriers, which are determined by their crystal/electronic structures in their pristine states and the structure evolution that accompanies variation of the lithium concentration.  Multiple studies of LLOs have been performed, especially to clarify the kinetics-controlled reaction processes associated with Li+ extraction/insertion behavior from/into the crystalline grains under different voltage ranges.  Both electrochemical and kinetics studies have revealed that the Li+ diffusion behavior in LLOs is clearly divided into several processes, thereby indicating possible inhomogeneous crystalline structures4-6.  Further studies that used in-situ X-ray absorption spectroscopy (XAS) have indicated that an Li2MnO3-like structure distributed in LLOs is a key factor that affects the rate capability5.  However, researchers do not yet understand the cause of the poor electrode kinetics of LLOs, which is challenging primarily because of their pristine structural complexities. 

Several studies that have investigated the structure of LLOs have been conducted7
,
8.  Delmas and Tarascon have revealed that the complicated local structure of LLOs can be attributed to rotation of the LiMn2-like layer in the Li2MnO3-like structure 9
,
10, which has also been demonstrated in our work8.  Furthermore, we have identified the coexistence of two structures (rhombohedral LiTMO2 structure with  space group and monoclinic Li2MnO3–like structure with C2/m space group) on the grain edge using advanced microscopy technology 8, which has been well validated by refining the synchrotron X-ray diffraction (SXRD) patterns 3.  However, the interior configuration of LLO materials, including direct observation of the Li2MnO3-like structure distribution in crystalline grain, structure- or composition-coordination environments, and the relationship with the Li+ migration behavior are still unclear, thus preventing further in-depth understanding of their electrode kinetics, structure evolution, cycle stability and reaction mechanisms.

Undoubtedly, it is imperative to gain a clear understanding of the Li2MnO3-like structure of a crystalline grain because it is the basis for understanding these compounds and improving their electrochemical performance. 

In this conference, based on cross-sectional thin transmission-electron-microscopy (TEM) specimens (CSTTs) ‘anatomized’ from Li1.2Mn0.567Ni0.166Co0.067O2 powders, the atomic-scale Li2MnO3-like structure distribution within multiple monocrystal-like domains and domain boundaries (DBs) in a single crystalline grain was revealed using a variety of microscopy techniques and computer simulations. The Li+ migration in the Li2MnO3-like structure along different directions with or without DBs and the effect of implantation of element segregation were thoroughly investigated using density functional theory (DFT). 

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