A Ni/Mn composite precursor was prepared as the precursor for Li-rich cathode materials under an air-precipitation atmosphere in a continuous stirred-tank reactor (CSTR). The nucleation and growth of precursor particles were investigated during the CSTR process by monitoring particle morphology, chemical composition, and phase composition. It was found that the particle shape and chemical composition were not homogeneous in the early stages of co-precipitation and did not become so until after 4 h. Mn2+ ion oxidation was verified to occur during the entire course of precipitation by X-ray diffraction. The final precipitate was identified as a two-phase system consisting of a Ni/Mn layered double hydroxide phase and a Mn3O4 phase. Scanning electron microscopy and an N-2 adsorption-desorption test revealed the porosity and surface area of the material. A Li-rich cathode material was synthesized using the final precipitate as a precursor; the electrochemical performance of this cathode material is reported in this article.