⁶Li MAS NMR study of lithium layered oxides containing Ni ²⁺ ions: Experiments and DFT calculations

^6,7Li MAS NMR spectra are sensitive to the presence of paramagnetic cations in the local environment of the lithium ions and is used to probe the local ordering in layered LiTi_0.5Ni_0.5O ₂ and Li[Ni_xMn_(2-x)/3O₂ phases. In the Li[Ni_xLi_1/3-2x/3Mn_2/3-x/3]O₂ phases, both Ni²⁺ and Mn⁴⁺ ions are paramagnetic and therefore interact with Li, leading to several NMR signals depending on the Li environment. In contrast, in LiTi_0.5⁴⁺Ni _0.5²⁺O₂, only the nickel ions are paramagnetic and interact with Li. Based on the intensities of the resonances, we find a tendency for Ni²⁺ to replace Li⁺ in the Li layers, so as to form "anti-sites", and a strong preference for Li in the metal layers to occupy sites near Mn⁴⁺ or Ti⁴⁺ ions. We also show that the Li⁺/Ni²⁺ cation disordering in LiTi _0.5Ni_0.5O₂ strongly depends on the synthesis conditions. We also show that the metal ions in the metal layers are much more disordered in LiTi_0.5Ni_0.5O₂ than in LiMn _0.5Ni_0.5O₂.