Lithium-rich layered oxides (LLOs) have been extensively studied as cathode materials for lithium-ion batteries (LIBs) by researchers all over the world in the past decades due to their high specific capacities and high charge-discharge voltages. However, as cathode materials LLOs have disadvantages of significant voltage and capacity decays during the charge-discharge cycling. It was shown in the past that fine-tuning of structures and compositions was critical to the performances of this kind of materials. In this report, LLOs with target composition of Li_1.17Mn_0.50Ni_0.24Co_0.09O₂ were prepared by carbonate co-precipitation method with different pH values. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM), and electrochemical impedance spectroscopies (EIS) were used to investigate the structures and morphologies of the materials and to understand the improvements of their electrochemical performances. With the pH values increased from 7.5 to 8.5, the Li/Ni ratios in the compositions decreased from 5.17 to 4.64, and the initial coulombic efficiency, cycling stability and average discharge voltages were gained impressively. Especially, the material synthesized at pH = 8.5 delivered a reversible discharge capacity of 263 mAh·g^-1 during the first cycle, with 79.0% initial coulombic efficiency, at the rate of 0.1 C and a superior capacity retention of 94% after 100 cycles at the rate of 1 C. Furthermore, this material exhibited an initial average discharge voltage of 3.65 V, with a voltage decay of only 0.09 V after 50 charge-discharge cycles. The improved electrochemical performances by varying the pH values in the synthesis process can be explained by the mitigation of layered-to-spinel phase transformation and the reduction of solid-electrolyte interface (SEI) resistance. We hope this work can shed some light on the alleviation of voltage and capacity decay issues of the LLOs cathode materials.