Sequential delithiation behavior and structural rearrangement of a nanoscale composite-structured Li_1.2Ni_0.2Mn_0.6O₂...

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_1.2Ni_0.2Mn_0.6O₂ 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 LiNi_0.25Mn_0.75O₂. After the structural activation, the Li ions were reversibly extracted from the newly formed domainless structure.