Superlattice ordering of Mn, Ni, and Co in layered alkali transition metal oxides with P2, P3, and O3 structures

Na2/3Ni1/3Mn2/3O2 adopts a layered structure (P2) where the Ni and Mn cations form an ordered arrangement with a root 3a x root 3a superlattice. The ordered superlattice is preserved in T2-structure Li2/3Ni1/3Mn2/3O2 prepared from the Na-containing parent phase by ion exchange. In attempts to improve the electrochemical properties of Li2/3Ni1/3Mn2/3O2, we have investigated the effect of Co substitution for Ni in the parent phase. Using X-ray and neutron diffraction, it is shown that increasing amounts of Co first weaken and then suppress the superlattice ordering in the transition metal layers. For x less than or equal to 1/12 in Na-2/3 [CoxNi1/3-xMn2/3]O-2 the superlattice persists, but for x greater than or equal to 1/6 the superlattice is not observed. During ion exchange to make lithium-containing materials, only samples with well-developed superlattices result in crystalline products; otherwise, a stacking-faulted O2-type structure is obtained. The ordering of transition metals in transition metal layers was also found in P3-Na-2/3[Ni1/3Mn2/3] O-2 and O3-Li-2/3 [Ni1/3Mn2/3]O-2, but not in O3-Na[Ni1/2Mn1/2]O-2. The necessary conditions for superlattice ordering of transition metal atoms in the transition metal layers are discussed.