Superlattice Ordering of Mn, Ni, and Co in Layered Alkali Transition Metal Oxides with P2, P3, and O3 Structures

Abstract

Na 2/3 Ni 1/3 Mn 2/3 O 2 adopts a layered structure (P2) where the Ni and Mn cations form an ordered arrangement with a √3a × √3a superlattice. The ordered superlattice is preserved in T2-structure Li 2/3 Ni 1/3 Mn 2/3 O 2 prepared from the Na-containing parent phase by ion exchange. In attempts to improve the electrochemical properties of Li 2/3 Ni 1/3 Mn 2/3 O 2, 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 ≤ 1/12 in Na 2/3 [Co x Ni 1/3 - x Mn 2/3 ]O 2 the superlattice persists, but for x ≥ 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 [Ni 1/3 Mn 2/3 ]O 2 and O3−Li 2/3 [Ni 1/3 Mn 2/3 ]O 2, but not in O3−Na[Ni 1/2 Mn 1/2 ]O 2 . The necessary conditions for superlattice ordering of transition metal atoms in the transition metal layers are discussed.