| Summary: | The layered oxides LiNi<sub>x</sub>Mn<sub>y</sub>Co<sub>z</sub>O<sub>2</sub> (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<sup>+</sup>/Ni<sup>2+</sup> cation disordering originating from the proximity of the Li<sup>+</sup> and Ni<sup>2+</sup> ionic radii. We synthesized a series of the LiNi<sub>0</sub>.<sub>8</sub>Mn<sub>0</sub>.<sub>1</sub>Co<sub>0</sub>.<sub>1</sub>O<sub>2</sub> 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 <i>I</i><sub>003</sub>/<i>I</i><sub>104</sub> integral intensity ratio and <i>c</i>/(2√6<i>a</i>) ratio of pseudocubic subcell parameters, was verified against the refined defect concentrations. The <i>I</i><sub>003</sub>/<i>I</i><sub>104</sub> ratio can serve as a quantitative measure of g(Ni<sub>Li</sub>) only after explicit correction of intensities for preferred orientation. Being normalized by the total scattering power of the unit cell, the <i>I</i><sub>003</sub>/<i>I</i><sub>104</sub> ratio depends linearly on g(Ni<sub>Li</sub>) for each disordering scheme. The <i>c</i>/(2√6<i>a</i>) ratio appears to be not reliable and cannot be used for a quantitative estimate of g(Ni<sub>Li</sub>). In turn, the volume of the <i>R</i><inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mover accent="true"><mn>3</mn><mo>¯</mo></mover></semantics></math></inline-formula><i>m</i> unit cell correlates linearly with g(Ni<sub>Li</sub>), 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<sup>+</sup>/Ni<sup>2+</sup> 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<sup>+</sup>/Ni<sup>2+</sup> mixing defects in Ni-rich NMCs.
|
|---|