Lithium transition metal (M) oxides with a rhombohedral structure, r-LiMO2, have attracted a great deal of attention as a positive electrode material for lithium-ion batteries. Despite intensive studies thus far, Mn-rich r-LiMO2 compounds have remained unattainable, due to a cooperative Jahn-Teller distortion of Mn3+ ions in the MnO6 octahedra. We employed a high-pressure method for synthesizing r-LiCo xMn1- xO2 ( r-LCMO) with x = 0.5 and examined its electrochemical properties in a nonaqueous lithium cell. The high-pressure method successfully suppressed the Jahn-Teller distortion of Mn3+ ions, and the r-LCMO phase was observed in a wide temperature-pressure region when using a LiOH·H2O precursor. The rechargeable capacity of the sample synthesized at 600 °C and 12 GPa reached 126 mAh g-1, although the r-LCMO phase was contaminated with electrochemically inactive rock-salt LCMO and hexagonal LCMO phases. Compositional and structural analyses clarified that the actual Co/Mn ratio of the r-LCMO phase was 64/36, which deviated slightly from the initial composition (50/50). The high-pressure method was found to be effective for synthesizing Mn-rich r-LiMO2 compounds, although their electrochemical properties should be improved.