Cobalt-free Li-Mn-rich layered oxides (LMLOs) are regarded as promising cathode materials for advanced lithium ion batteries (LIBs) because of their high discharge specific capacities (> 250 mAh g^-1). However, the formation mechanism during the synthesis of these oxides has not been fully understood, i.e. how are lithium and oxygen incorporated into the matrix architecture of the Li-free precursor on the process of high-temperature solid-state reaction? Herein, in situ high-temperature synchrotron radiation diffraction experiments were performed to monitor the structural evolution of the precursor combined with lithium source during thermal treatment. The in situ high-temperature diffraction results show that, with an increase in heating temperature, lithium and oxygen continuously enter into the spinel host matrix (Fd-3m) formed from the Li-free precursor, thus gradually causing the production of a halite-type phase (Fm-3m) and eventually results in the formation of a monoclinic layered phase (C2/m). These findings not only provides valuable information for industrial operation, but also contributes to a deeper understanding of the correlation between the synthesis, structure and electrochemical performances in LMLOs.