On the Elusive Crystallography of Lithium-Rich Layered Oxides: Novel Structural Models

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_1.28 Mn_0.54 Ni_0.13 Co_0.02 Al_0.03 O₂ , 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_1.28 Mn_0.54 Ni_0.13 Co_0.02 Al_0.03 O₂ by using the Rietveld refinement method considering the R $\overline{3}$ m 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 R $\overline{3}$ m structure. To summarize, two possible models able to reconcile the description of experimental data are proposed here for the structure of Li_1.28 Mn_0.54 Ni_0.13 Co_0.02 Al_0.03 O₂ : namely a monoclinic C2/m defective lattice (prototype Li₂ MnO₃ ) and a monoclinic defective supercell derived from the rhombohedral R $\overline{3}$ m unit cell (prototype LiCoO₂ ).