<p>Lithium-rich manganese cathode materials are regarded as next-generation lithium-ion battery cathodes due to their high theoretical specific capacity and low cost. However, some disadvantages such as low initial coulombic efficiency, voltage decay and poor rate performance hinder their commercial application. In this study, a series of lithium-rich manganese materials with the general formula 0.5Li₂MnO₃·0.5LiNi_<i>x</i>Co_{(2/3-<i>x</i>)}Mn_1/3O₂ is designed and synthesized for <i>x</i> = 1/3, 4/9, 5/9 and 2/3. The effect of the nickel–cobalt ratio on the structure and performance is studied and the results show that the increasing of Ni content favors the suppression of the Mn⁴⁺/Mn³⁺ reaction occurring in the low-voltage region, enabling the material to exhibit a higher discharge voltage and a smaller voltage decay rate, but the synergistic effect of Co is also important. 0.5Li₂MnO₃·0.5LiNi_5/9Co_1/9Mn_1/3O₂ (LNCMO-513) is identified as the optimal composition with high discharge voltage and long cycle life. Although the initial 0.1C discharge capacity of LNCMO-513 is slightly lower than 0.5Li₂MnO₃·0.5LiNi_1/3Co_1/3Mn_1/3O₂ (LNCMO-333), the mid-discharge voltage increases from 3.58 to 3.73&#xa0;V. After 200 cycles at 1 C, the capacity of LNCMO-513 remains at 163.6&#xa0;mAh/g with a capacity retention rate of 90.44%, whereas LNCMO-333 exhibits only 146.2&#xa0;mAh/g and 73.62%. Notably, the initial mid-discharge voltages at 1C for LNCMO-513 exhibits 3.62&#xa0;V and an average voltage decay rate of 1.10&#xa0;mV/cycle over 200 cycles, while these two values for LNCMO-333 are only 3.40&#xa0;V and 2.01&#xa0;mV/cycle, significantly mitigating the severe voltage decay drawback common in lithium-rich materials. Furthermore, LNCMO-513 exhibits outstanding rate performance with an average discharge capacity of 121.4 mAh/g and an average mid-discharge voltage of 3.31&#xa0;V at 5C, which is more greatly excessive than 55.28&#xa0;mAh/g and 2.75&#xa0;V of LNCMO-333.</p>