Jahn-Teller mediated ordering in layered LixMO2 compounds

Lithium ordering in layered LixM O-2 compounds (M = Ni, Co, Mn) is investigated with first-principles calculations. We found that apart from the in-plane Li-Li interactions, the stability of ordered LixM O-2 structures strongly depends on the interlayer Li-Li interactions through the M cations. When several stacking sequences are possible, the selection of stacking is shown to be driven by the electronic structure of the transition-metal cations (metallic for Co+3/+4, localized for Ni+3/+4 and Mn+3/+4). In LixCoO2 the stacking is chosen so as to minimize the variations in coordination of the symmetrically different Co sites, thus enhancing electronic charge delocalization. In LixNiO2, lithium ordering couples to orbital and Jahn-Teller ordering so as to form 180 degrees Li-O-Ni+3-O-Li complexes. The presence of these 180 degrees Li-A-O-M-O-Li-B interactions ensures distinct M environments and charge localization. We indicate how the coupling between the e(g)* and Li-2s orbital hybridization and the Jahn-Teller distortion produces a long-range attractive interaction between Li ions.