Jahn-Teller mediated ordering in layeredLixMO2compounds

Abstract

Lithium ordering in layered ${\mathrm{Li}}_{x}M{\mathrm{O}}_2$ compounds $(M=\mathrm{N}\mathrm{i},\mathrm{}\mathrm{C}\mathrm{o},\mathrm{}\mathrm{Mn})$ is investigated with first-principles calculations. We found that apart from the in-plane Li-Li interactions, the stability of ordered ${\mathrm{Li}}_{x}M{\mathrm{O}}_2$ 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 ${\mathrm{Co}}^{+3/+4\mathrm{}},$ localized for ${\mathrm{Ni}}^{+3/+4\mathrm{}}$ and ${\mathrm{Mn}}^{+3/+4\mathrm{}}).\mathrm{}$ In ${\mathrm{Li}}_x{\mathrm{CoO}}_2$ the stacking is chosen so as to minimize the variations in coordination of the symmetrically different Co sites, thus enhancing electronic charge delocalization. In ${\mathrm{Li}}_x{\mathrm{NiO}}_2,$ lithium ordering couples to orbital and Jahn-Teller ordering so as to form 180° ${\mathrm{L}\mathrm{i}-\mathrm{O}-\mathrm{N}\mathrm{i}}^{+3\mathrm{}}-\mathrm{O}-\mathrm{L}\mathrm{i}$ complexes. The presence of these 180° ${\mathrm{Li}}_A-\mathrm{O}-M-{\mathrm{O}-\mathrm{L}\mathrm{i}}_B$ interactions ensures distinct M environments and charge localization. We indicate how the coupling between the $e_g^{*}$ and $\mathrm{Li}-2s$ orbital hybridization and the Jahn-Teller distortion produces a long-range attractive interaction between Li ions.