The layered oxides LiNi_xMn_yCo_zO₂ (NMCs, x + y + z = 1) with high nickel content (x ≥ 0.6, Ni-rich NMCs) are promising high-energy density-positive electrode materials for Li-ion batteries. Their electrochemical properties depend on Li⁺/Ni²⁺ cation disordering originating from the proximity of the Li⁺ and Ni²⁺ ionic radii. We synthesized a series of the LiNi₀.₈Mn₀.₁Co₀.₁O₂ NMC811 adopting two different disordering schemes: Ni for Li substitution at the Li site in the samples finally annealed in air, and close to Ni↔Li antisite disorder in the oxygen-annealed samples. The defect formation scenario was revealed with Rietveld refinement from powder X-ray diffraction data, and then the reliability of semi-quantitative parameters, such as I₀₀₃/I₁₀₄ integral intensity ratio and c/(2√6a) ratio of pseudocubic subcell parameters, was verified against the refined defect concentrations. The I₀₀₃/I₁₀₄ ratio can serve as a quantitative measure of g(Ni_Li) only after explicit correction of intensities for preferred orientation. Being normalized by the total scattering power of the unit cell, the I₀₀₃/I₁₀₄ ratio depends linearly on g(Ni_Li) for each disordering scheme. The c/(2√6a) ratio appears to be not reliable and cannot be used for a quantitative estimate of g(Ni_Li). In turn, the volume of the R$\overline{3}$m unit cell correlates linearly with g(Ni_Li), at least for defect concentrations not exceeding 5%. The microscopy techniques such as high-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and electron diffraction tomography (EDT) allow us to study the materials locally, still, there is no proper quantitative approach for comprehensive analysis of defects. In the present work, the TEM-assisted quantitative Li⁺/Ni²⁺ disordering analysis with EDT and HAADF-STEM in six Ni-rich NMC samples with various defects content is demonstrated. Noteworthy, while PXRD and EDT methods demonstrate overall defect amounts, HAADF-STEM allows us to quantitatively distinguish regions with various disordering extents. Therefore, the combination of mentioned PXRD and TEM methods gives the full picture of Li⁺/Ni²⁺ mixing defects in Ni-rich NMCs.

The review discusses the complex properties of nickel and its role as a critical element for ensuring a confident transition to a new technological paradigm from fossil fuels in favor of using advanced electrochemical storage and energy conversion systems. The main classes of nickel-containing materials of the positive electrode (cathode) for metal-ion batteries are discussed, the place of nickel among other 3d-metals used in the industry of electrochemical energy storage is determined. The main methods and approaches for the synthesis of state-of-the-art and next generation cathode materials based on layered Ni-containing oxides are presented. The crystal and electronic structures of these materials, including their evolution in the process of (de)intercalation of alkali metal cations, are considered in the context of their electrochemical properties. The most acute problems facing modern materials science on the way to commercialization and industrial production of new generation highenergy density cathode materials are determined. At the end of the review, promising directions for the further development of nickelcontaining cathode materials are outlined. The bibliography includes 252 references.