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

Na2/3Ni1/3Mn2/3O2 adopts a layered structure (P2) where the Ni and Mn catio ns form an ordered arrangement with a root 3a x root 3a superlattice. The o rdered superlattice is preserved in T2-structure Li2/3Ni1/3Mn2/3O2 prepared from the Na-containing parent phase by ion exchange. In attempts to improv e 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 super lattice persists, but for x greater than or equal to 1/6 the superlattice i s not observed. During ion exchange to make lithium-containing materials, o nly samples with well-developed superlattices result in crystalline product s; otherwise, a stacking-faulted O2-type structure is obtained. The orderin g 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 me tal atoms in the transition metal layers are discussed.