Li1+x (Ni0.37Mn0.63)1−x O2 (x =0.123, 0.111, 0.086, 0.070, 0.031) cathode materials were synthesized via coprecipitation of carbonates and the samples with long lifespan for lithium ion batteries were obtained through adjusting the content of lithium. Their crystal structure and electrochemical performance were characterized by means of powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), inductive coupled plasma (ICP), galvanostatic charge/discharge test and electrochemical impedance spectrum (EIS). The XRD data show that the materials can be indexed as a single α-NaFeO2 phase except the sample with x =0.031. The FESEM results indicate that the primary particles size increase with an increase of x value and the secondary particles retain the spherical morphology. The as-prepared sample with x=0.086 delivers the largest discharge capacity of 232.1mAhg−1 and a high initial efficiency of 81.8% at 0.2C in the potential range of 2.5–4.7V. Moreover, the better cycle performances are obtained for samples with x =0.123 and 0.111, and the capacity retentions are up to 89 and 81% of the first discharge capacity at 0.5C after 500 cycles, respectively.