Recently, orthorhombic structured $LiMnO_2$ has been studied as a potential cathode material of Li ion secondary battery. It has high theoretical capacity (285mAh/g), low cost and environmental affinity. In spite of these merits, the big obstacle to its commercialization is the phase transition, i.e. structural instability, from orthorhombic $LiMnO_2$ (Pmnm, zig-zag layered structure) to $LiMn_2O_4$ (Fd3m, spinel structure). Because $Mn^{3+}$ ions induce Jahn-Teller distortion, the orthorhombic $LiMnO_2$ with zigzag layered structure is a thermodynamically unstable phase. Furthermore, during electrochemical $Li^+$ insertion/desertion cycling, phase transition to spinel occurs due to the migration of cations; manganese ions and lithium ions migrate into the octahedral sites and tetrahedral sites in lithium layers, repectively. Orthorhombic $LiMnO_2$ has a small initial discharge capacity below 50mAh/g and needs much time to reach maximum capacity. Therefore, in order to commercialize the $LiMnO_2$ as a cathode material, phase transition must be prevented. For that purpose, $LiMnO_2$ should have a perfect-layered structure instead of the meta-stable zigzag layered structure by lowering the concentration of $Mn^{3+}$ ion in $LiMnO_2$. In this work, to reduce the concentration of $Mn^{3+}$ ions, 50at% of divalent nickel $(Ni^{2+})$ ions are substituted by using a sol-gel method, which can produce uniform and homogeneous powders. It is considered that the $LiMn_{0.5}Ni_{0.5}O_2$ powder has perfect layer structure of R-3m symmetry because all manganese ions are tetravalent. $(Mn^{4+})$ The $LiNi_{0.5}$ $Mn_{0.5}O_2$ has a discharge capacity of 100mAh/g and stable cyclic performance up to 50 cycles. By X-ray reflection diffraction (XRD) and Cyclic voltammetry (CV) analyses, it is confirmed that $LiNi_{0.5}Mn_{0.5}O_2$ shows no phase transition during electrochemical cycling. However, 50at% substitution of nickel deteriorates the thermal stability of lithium manganese oxide. Differential scanning calorimeter (DSC) analysis shows that the exothermic temperature corresponding to the oxygen evolution is lowered from 247℃ $(LiMnO_2)$ to 232℃ $(LiNi_{0.5}Mn_{0.5}O_2)$. In general, lithium nickel oxide has much less thermal stability than other cathode materials such as $LiCoO_2$ and $LiMn_2O_4$. To enhance the thermal stability, other elements, such as Al and Ti, which have the strong bonding energy with oxygen have been doped. In $LiNi_{0.5}Mn_{0.5}O_2$ small amount of Al is doped. It is found that 5at% of Al is the optimum composition. There are no second phase and capacity-fading phenomenon. Over 5at% Al, the discharge capacity faded with the formation of second phase. The first discharge capacity of $LiMn_{0.5}Ni_{0.45}Al_{0.05}O_2$ is 100mAh/g. The cyclic performance is similar to that of $LiMn_{0.5}Ni_{0.5}O_2$. finally, the thermal stability of $LiMn_{0.5}Ni+{0.45}Al_{0.05}O_2$ has increased; the exothermic temperature increases from 232 $(LiNi_{0.5}Mn_{0.5}O_2)$ to 248℃.
