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Lithium-rich layered oxides (LRLOs) are one of the most attractive families among future positive electrode materials for the so-called fourth generation of lithium-ion batteries (LIBs). Their electrochemical performance is enabled by the unique ambiguous crystal structure that is still not well understood despite decades of research. In the literature, a clear structural model able to describe their crystallographic features is missing thereby hindering a clear rationalization of the interplay between synthesis, structure, and functional properties. Here, the structure of a specific LRLO, Li_1.28Mn_0.54Ni_0.13Co_0.02Al_0.03O₂, using synchrotron X-ray diffraction (XRD), neutron diffraction (ND), and High-Resolution Transmission Electron Microscopy (HR-TEM), is analyzed. A systematic approach is applied to model diffraction patterns of Li_1.28Mn_0.54Ni_0.13Co_0.02Al_0.03O₂ by using the Rietveld refinement method considering the Rm and C2/m unit cells as the prototype structures. Here, the relative ability of a variety of structural models is compared to match the experimental diffraction pattern evaluating the impact of defects and supercells derived from the Rm structure. To summarize, two possible models able to reconcile the description of experimental data are proposed here for the structure of Li_1.28Mn_0.54Ni_0.13Co_0.02Al_0.03O₂: namely a monoclinic C2/m defective lattice (prototype Li₂MnO₃) and a monoclinic defective supercell derived from the rhombohedral Rm unit cell (prototype LiCoO₂).