Sequential delithiation behavior and structural rearrangement of a nanoscale composite-structured Li1.2Ni0.2Mn0.6O2 during charge–discharge cycles
Keiji Shimoda, Koji Yazawa, Toshiyuki Matsunaga, Miwa Murakami, Keisuke Yamanaka, Toshiaki Ohta, Eiichiro Matsubara, Zempachi Ogumi & Takeshi Abe
Scientific Reports volume 10, Article number: 10048 (2020) Cite this article

[Abstract]
Lithium- and manganese-rich layered oxides (LMRs) are promising positive electrode materials for next-generation rechargeable lithium-ion batteries. Herein, the structural evolution of Li1.2Ni0.2Mn0.6O2 during the initial charge–discharge cycle was examined using synchrotron-radiation X-ray diffraction, X-ray absorption spectroscopy, and nuclear magnetic resonance spectroscopy to elucidate the unique delithiation behavior. The pristine material contained a composite layered structure composed of Ni-free and Ni-doped Li2MnO3 and LiMO2 (M?=?Ni, Mn) nanoscale domains, and Li ions were sequentially and inhomogeneously extracted from the composite structure. Delithiation from the LiMO2 domain was observed in the potential slope region associated with the Ni2+/Ni4+ redox couple. Li ions were then extracted from the Li2MnO3 domain during the potential plateau and remained mostly in the Ni-doped Li2MnO3 domain at 4.8?V. In addition, structural transformation into a spinel-like phase was partly observed, which is associated with oxygen loss and cation migration within the Li2MnO3 domain. During Li intercalation, cation remigration and mixing resulted in a domainless layered structure with a chemical composition similar to that of LiNi0.25Mn0.75O2. After the structural activation, the Li ions were reversibly extracted from the newly formed domainless structure.

[Introduction]
The world is moving towards electrification as CO2 emission standards have resulted in a growing battery market. Rechargeable lithium ion batteries (LIBs) have been widely used as a power source for portable devices and currently the global market for electric vehicles (EVs) demands higher power, higher energy density, longer life, and lower cost batteries. A significant amount of effort has been dedicated to the development of improved battery materials, with recent studies investigating lithium- and manganese-rich layered oxides (LMRs), which are preferred candidate materials for the next-generation LIB positive electrodes due to their high reversible capacities of ≥200?mA?h g–1 at 2.0–4.8 V1,2,3,4,5,6,7,8,9,10. In contrast, several drawbacks have been reported including capacity fading and voltage decay during long-term charge–discharge cycles6,7,8,9,10,11,12,13,14. The structural factors resulting in these problems must be solved to develop stable and high power batteries.

LMRs exhibit a layered rock-salt structure commonly represented as Li[Li(1–2×)/3MxMn(2–x)/3]O2 (M?=?Ni, Co, etc.) in single-phase notation or as xLi2MnO3·(1–x)LiMO2 in composite notation. Based on single-phase notation, the crystal structure can be indexed with a space group of C2/m, where the structure is homogeneous and Li ions occupy the Li layer and part of the transition-metal (TM) layer with intralayer ordering between the Li and TM ions. In contrast, the composite notation can be expressed as a mixture of C2/m and R–3?m structures, where Li2MnO3 (alternatively expressed as Li[Li1/3Mn2/3]O2) and LiMO2 exhibit common d spacing. These two phases are dispersed as nanoscale domains over the entire structure15. Unfortunately, X-ray diffraction (XRD) studies provided no clear indication of which structural model appropriately represents the real structure. Many atomic column observations using advanced scanning transmission electron microscopy (STEM) have provided evidence of two structural domains in the composite structure16,17,18,19, while other reports showed a homogeneous atomic column, suggesting a single-phase structure20,21,22. Solid-state nuclear magnetic resonance (NMR) spectroscopy is sensitive to cation substitution in the first and second cation coordination shells, allowing the local structure to be examined at a length scale of <5?Å in diameter. Grey et al. reported the nonrandom cation distribution around Li ions in Li[Li(1–2×)/3MxMn(2–x)/3]O2 via 6Li magic-angle spinning (MAS) NMR analyses, implying a composite nature instead of homogeneous solid solution23,24,25.

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