Comprehensive Study of Li⁺/Ni²⁺ Disorder in Ni-Rich NMCs Cathodes for Li-Ion Batteries

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.