Well-defined orthorhombic $LiMnO_2;and;LiCo_{0.1}Mn_{0.9}O_2$ were synthesized by a solid-state reaction and quenching process. X-ray diffraction (XRD) results revealed that the as-synthesized powders showed an orthorhombic phase of a space group with Pmnm. The $Li/LiMnO_2;and;Li/LiCo_{0.1}Mn_{0.9}O_2$ cells were constituted and cycled galvanostatically in the voltage range of 2.0-4.3 V vs. $Li/Li^+$ at a current density of $0.5;mA;cm^{-2}$ at room temperature and $50^{circ}C$, respectively. The results demonstrated that the highest specific capacity of $Li/LiMnO_2$ cells at room temperature and $50^{circ}C$ was 95 and $155;mAh;g^{-1}$, respectively. As for $Li/LiCo_{0.1}Mn_{0.9}O_2$ cells, the highest specific capacity at room temperature and $50^{circ}C$ was 160 and $250;mAh;g^{-l}$, respectively. It could be seen that the performance of $Li/LiCo_{0.1}Mn_{0.9}O_2$ cells was better than that of $Li/LiMnO_2$ cells.