The effect of the synthesis temperature on the chemical composition of "Li1.20Mn0.54Co0.13Ni0.13O2" was considered using thermogravimetric analyses (TGA) and in situ X-ray diffraction (XRD) during thermal treatment. A continuous and small weight loss is observed above 800 °C because of Li evaporation, and the lamellar phase disappears to the benefit of a spinel-type phase formed above 940 °C. The layered structure is recovered upon cooling under air. "Li1.20Mn0.54Co0.13Ni0.13O2" materials synthesized at 800, 900, and 1000 °C show very similar compositions, structures, and electrochemical properties despite very different crystallization states. Their average structure is α-NaFeO2-type and described in the R3̅m space group, with less than 0.02 Ni2+ ions in the Li site. This peculiar composition "Li1.20Mn0.54Co0.13Ni0.13O2", with one-third of large cations (Li+, Ni2+) and two-thirds of small cations (Mn4+, Co3+) promotes the extension of the cation ordering in the slabs as revealed by the √3ahex. × √3ahex. superstructure, but without full correlation between the ordered slabs along the chex. stacking axis. Neutron and electron diffraction associated with NMR and Raman spectroscopies are shown to be efficient tools to get more insights into the average and local structures of these complex layered materials.