Monodisperse spherical Mn0.75Ni0.25(OH)2 precursors built up from plate-like primary particles have been successfully synthesized by the control of pH values during a co-precipitation reaction. The size of spherical particles, namely the secondary particles, is observed to decrease with increasing pH value from 9.0 to 11.0, and is accompanied by a series of shape changes of the primary particles from close-packed plates to well-exposed nanoplates, and then to nanoparticles. Further lithiation of these hydroxide precursors produces the final lithium-rich layered Li1.2Mn0.6Ni0.2O2 cathode materials without destroying the morphology of the precursors. Electrochemical measurements show that the spherical cathode material assembled from well-exposed nanoplates exhibits superior rate capability and good cyclability compared to other electrode materials, which can be attributed to its uniform particle size and the favorable shape which facilitates the diffusion of lithium ions. Through the control of the sample morphologies, we provide a simple and effective way to enhance the lithium storage capability of lithium-rich layered oxide cathode materials for high-performance lithium-ion batteries.