A series of Co (x) Mn3-x sulfides (x = 0, 0.5, 1, 1.5, 2, 2.5, and 3) were prepared by a facile one-step solvent thermal route. The crystal structure, morphology, and electrochemical performance of Co-Mn sulfide can be tuned successfully by varying different Co/Mn ratios. As the Co/Mn ratio increases, the electrochemical performance is improved, which can be ascribed to the synergistic effect between Co ions and Mn ions. Co2.5Mn0.5 sulfide exhibits the highest specific capacity (289 C g(-1) at a current density of 1 A g(-1)), together with great rate capability (54.6% retention when current density increases from 1 to 50 A g(-1)) and excellent cycling stability (95.1% of initial value can be retained after 2000 cycles). Moreover, an asymmetric device using Co2.5Mn0.5 sulfide as positive electrode, and RGO nanosheets as negative electrode shows high energy density of 22.3 Wh kg(-1) at a power density of 750 W kg(-1), as well as excellent cycling performance with 85% retention after 5000 cycles.