Cation Ordering in Layered O3 Li[Ni_xLi_1/3-2x/3Mn_2/3-x/3]O₂ (0 ≤ x ≤ ¹/₂) Compounds

Abstract

Three layered compounds, Li[Ni1/2Mn1/2]O2 (x = 1/2), Li[Li1/9Ni1/3Mn5/9]O2 (x = 1/3), and Li2MnO3 (x = 0) of the Li[NixLi1/3-2x/3Mn2/3-x/3]O2 series were studied by electron diffraction in combination with synchrotron X-ray powder diffraction analyses. In-plane √3ahex. × √3ahex. ordering was found in all three samples, and a likely model for the occupancies of Li+, Ni2+, and Mn4+ on the sublattices of the transition metal layer was proposed. Perpendicular to the chex. axis, the ordered transition metal layers in the layered Li[Ni1/2Mn1/2]O2 and Li[Li1/9Ni1/3Mn5/9]O2 crystals could be arranged mostly in the P3112 stacking with some C2/m stacking (such as abab...) abnormally. It is believed that the nickel ions in the lithium layer of these two samples play an important role in the stacking of transition metal layers. Moreover, electron diffraction analyses suggested that individual Li2MnO3 crystals that were obtained at 850 °C had at least two different C2/m stacking sequence variants (abab..., caca..., and cbcb...) coexisting along the chex. axis, which were associated with the presence of some P3112 stacking abnormally.