Lithium-rich layered oxide, Li[Li0.131Ni0.304Mn0.565]O2 with uniform particle size, is prepared through co-precipitation of metal hydroxide with synergistic dispersion of polyvinylpyrrolidone and ethylene glycol and subsequent solid reaction with lithium hydroxide. The crystal structure and morphology of the prepared sample is characterized with X-ray diffraction and scanning electron microscope. It is found that the prepared sample is a solid solution of Li2MnO3 and LiNi0.5Mn0.5O2, with uniform particle size of about 230 nm. Charge/discharge tests indicate that the prepared sample exhibits improved elevated-temperature discharge capacity and cycling stability. In the region of 3.0–4.8 V, the prepared sample delivers initially a 0.1 C (1 C = 263 mAh g−1) capacity of 256 mAh g−1 at 50 °C and 205 mAh g−1 at 25 °C, and the 0.1 C capacity at 25 °C retains 178 mAh g−1 after 60 cycles. The capacity decay mechanism is understood in detail through dQ/dV analyses.