Electrochemical and structural properties of Li-rich layered Li1.2Ni0.2Mn0.6O2 cathode materials have been investigated by measurements of rate capability, chemical diffusion coefficient of Li+, synchrotron X-ray diffraction, and neutron diffraction. Faster initial charging enhanced the rate capability and the chemical diffusion of Li+ in solid. In the X-ray and the neutron diffraction patterns, the peaks related to the distance of interlayer, such as (003) and (108) in the rhombohedral R 3 ¯ m notation, broadened upon charging. Furthermore, increasing the initial charging rate decreased this broadening of the peaks. These results indicate that structural stabilization plays an important role in diffusion of Li+ in Li1.2Ni0.2Mn0.6O2, and leads to the high rate capability.