Sequential delithiation behavior and structural rearrangement of a nanoscale composite-structured Li(1.2)Ni(0.2)Mn(0.6)O(2) during charge–discharge cycles

Lithium- and manganese-rich layered oxides (LMRs) are promising positive electrode materials for next-generation rechargeable lithium-ion batteries. Herein, the structural evolution of Li(1.2)Ni(0.2)Mn(0.6)O(2) 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 Li(2)MnO(3) and LiMO(2) (M = Ni, Mn) nanoscale domains, and Li ions were sequentially and inhomogeneously extracted from the composite structure. Delithiation from the LiMO(2) domain was observed in the potential slope region associated with the Ni(2+)/Ni(4+) redox couple. Li ions were then extracted from the Li(2)MnO(3) domain during the potential plateau and remained mostly in the Ni-doped Li(2)MnO(3) 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 Li(2)MnO(3) domain. During Li intercalation, cation remigration and mixing resulted in a domainless layered structure with a chemical composition similar to that of LiNi(0.25)Mn(0.75)O(2). After the structural activation, the Li ions were reversibly extracted from the newly formed domainless structure.