[en] Highlights: • Solid-state synthesis of Lithium-manganese-rich cathode is investigated by in situ XRD. • Covariance analysis and Rietveld refinement are used to analyze the HEXRD data. • A two-step process was adopted to minimize the elemental heterogeneity of the final product. • A substantial improvement on the reversible specific capacity for the material synthesized through the two-step process. 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[Li_0.2Mn_0.54Ni_0.13Co_0.13]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.