Lithium-manganese-rich transition metal oxides have attracted substantial R&D attention due to their potential for high energy-density lithium-ion batteries. In this work, in situ high-energy X-ray diffraction was deployed to investigate the phase evolution during the solid-state synthesis of Li[Li0.2Mn0.54Ni0.13Co0.13]O2. A step-wise consumption of the starting materials was observed during the one-step heating process primarily due to the heterogeneous nature of the precursor. According to observations from in situ high-energy X-ray diffraction, a two-step process was adopted to minimize the elemental heterogeneity of the final product. The electrochemical characterization results showed a substantial improvement on the reversible specific capacity for the material synthesized through the two-step process.