The influence of synthesis temperature and time on the length (capacity) of the plateau region during first charge in the high-capacity lithium-rich layered oxide Li_1.2Mn_0.6Ni_0.2O₂ and on the reversible capacity during subsequent charge–discharge cycles has been systematically investigated. The samples were synthesized by firing a sol–gel precursor obtained at 450 °C at various temperatures (850–1000 °C) for 24 h and at the optimum temperature of 900 °C for 6–72 h. The maximum length of the plateau region during the first charge and, consequently the maximum reversible capacity were achieved with the sample fired at 900 °C for 24 h. In contrast, the sample fired at 1000 °C for 24 h does not show any plateau region. In-depth characterization by X-ray diffraction, aberration-corrected transmission electron microscopy, scanning electron microscopy, inductively coupled plasma analysis, and electrochemical charge–discharge measurements reveals that the actual lithium content in the synthesized samples, compositional inhomogeneities, and the presence of a single C2/m phase vs a C2/m + R3̅m two-phase mixture play a critical role in the length of the plateau region, while particle size and surface area play a minor role. The study demonstrates the benefits of the formation of a single-phase C2/m solid solution with a lithium content of at least 1.16 in order to maximize the discharge capacity.