Li- and Mn-rich layered transition metal oxides (LMLOs) have attracted much attention because of their high theoretical capacities containing both transition metal cation and oxygen anion redox processes. However, the poor electrical conductivity and slow lithium-ion diffusion under high voltage result in an undesirable rate performance, which limits the commercial application of LMLOs. In this work, we found that the types of lithium source (including LiOH and Li2CO3) changed the intermediate transformation process, which in turn controlled the microstructure and morphology of LMLOs, thereby affecting the electrochemical performance. The prepared Li-rich cathode using LiOH as the lithium source accelerates the decomposition of the precursors. The structure undergoes rock-salt to disordered layered phase conversion and finally converts to an ordered layered structure. With Li2CO3 as the lithium source, the structure undergoes a spinel-layer intermediate transition process. The stable spinel phase is difficult to completely convert to a layered phase and remains in the final product. The LMLOs with spinel-layer composition exhibit an excellent rate performance of 178 mA h g–1 at 5 C, which is 39.1 mA h g–1 higher than that of LMLOs prepared by using LiOH as the lithium source. This insight into the preparation of high-performance materials by controlling the intermediate transformation provides a reference for the synthesis of advanced electrode materials through the reasonable selection of raw materials.