In this manuscript, we report an extensive study of the physico-chemical properties of different samples of O3-NaMnO₂, synthesized by sol–gel and solid state methods. In order to successfully synthesize the materials by sol–gel methods a rigorous control of the synthesis condition has been optimized. The electrochemical performances of the materials as positive electrodes in aprotic sodium-ion batteries have been demonstrated. The effects of different synthesis methods on both structural and electrochemical features of O3-NaMnO${}_2$ have been studied to shed light on the interplay between structure and performance. Noticeably, we obtained a material capable of attaining a reversible capacity exceeding 180 mAhg${}^{-1}$ at 10 mAg${}^{-1}$ with a capacity retention >70% after 20 cycles. The capacity fading mechanism and the structural evolution of O3-NaMnO${}_2$ upon cycling have been extensively studied by performing post-mortem analysis using XRD and Raman spectroscopy. Apparently, the loss of reversible capacity upon cycling originates from irreversible structural degradations.

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).

Lithium-rich layered oxides (LRLO) are a wide class of innovative active materials used in positive electrodes in lithium-ion (LIB) and lithium–metal secondary batteries (LMB). LRLOs are over-stoichiometric layered oxides rich in lithium and manganese with a general formula Li_1+xTM_1−xO₂, where TM is a blend of transition metals comprising Mn (main constituent), Ni, Co, Fe and others. Due to their very variable composition and extended defectivity, their structural identity is still debated among researchers, being likely an unresolved hybrid between a monoclinic (mC24) and a hexagonal lattice (hR12). Once casted in composite positive electrode films and assembled in LIBs or LMBs, LRLOs can deliver reversible specific capacities above 220–240 mAhg⁻¹, and thus they exceed any other available intercalation cathode material for LIBs, with mean working potential above 3.3–3.4 V vs Li for hundreds of cycles in liquid aprotic commercial electrodes. In this review, we critically outline the recent advancements in the fundamental understanding of the physical–chemical properties of LRLO as well as the most exciting innovations in their battery performance. We focus in particular on the elusive structural identity of these phases, on the complexity of the reaction mechanism in batteries, as well as on practical strategies to minimize or remove cobalt from the lattice while preserving its outstanding performance upon cycling.