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Change of local structures for 0.5Li2MnO3–0.5LiMn1/3Ni1/3Co1/3O2 in first charge process of different rates

Idemoto, Yasushi, Sekine, Takuya, Ishida, Naoya, Kitamura, Naoto
Journal of materials science 2017 v.52 no.14 pp. 8630-8649
X-radiation, cations, cobalt, crystal structure, geometry, manganese, mixing, models
An investigation was carried out into the charging rate dependence of local structural changes in a layered 0.5Li₂MnO₃–0.5LiMn₁/₃Ni₁/₃Co₁/₃O₂ solid solution during the initial charging cycle. To clarify the mechanism involved in local atomic rearrangement, a pair distribution function (PDF) analysis was performed using the results of powder neutron diffraction and synchrotron X-ray total scattering measurements. First-principles calculations (VASP code) were used to determine the initial structure when performing the PDF analysis. The bond-length strain (λ) and the bond-angle strain (σ ²) for the optimized model were calculated following the PDF analysis in order to clarify the effect of the charging rate on the crystal structure distortion. Before charging, the distortion was small for MnO₆ octahedra compared to that for NiO₆ and CoO₆ octahedra. During charging at a rate of 1C, the MnO₆ octahedra experienced increasing distortion, whereas at 3C the CoO₆ octahedra became more distorted. In addition, when charging at 3C, the values of λ and σ ² increased for NiO₆ octahedra that had entered the Li layer as a result of cation mixing. This appeared to be related to whether the localized atom was Mn or Co within the average structure during charge process. It is thought that distortion occurs in MO₆ octahedra containing whichever element becomes localized, and this depends on the charging rate. This leads to the possibility that decreasing the fractional composition of the element that becomes localized may lead to reduced distortion and improved the cyclability.