In order to evaluate the effect of Co on the structural stability of Li[Ni0.5-xCo2xMn0.5-x]O-2 (x <= 0.05), Co-doped Li[Ni0.5-xCo2xMn0.5-x]O-2 samples were synthesized by coprecipitation method. The structural instability, electrochemical properties, reactivity with electrolyte, and thermal stability of the Li[Ni0.5-xCo2xMn0.5-x]O-2 (x <= 0.05) were investigated. Co-substituted Li[Ni0.5-xCo2xMn0.5-x]O-2 (x = 0, 0.025, 0.05) showed increase of the specific discharge capacity and improved rate capability compared to Li[Ni0.5Mn0.5]O-2. However, X-ray diffraction study of Li[Ni0.5-xCo2xMn0.5-x]O-2 (charged at 4.5 V) showed that Co substitution increases the lattice parameter in the c axis dramatically, compared to the Co undoped electrode (Li[Ni0.5Mn0.5]O-2). Electron diffraction analysis confirmed that cobalt substitution induced significant structural transformation during cycling in the high-potential range (4.5 V). Furthermore, increase of Co amount in the Li[Ni0.5-xCo2xMn0.5-x]O-2 (x <= 0.05) accelerated Co dissolution at high-temperature storage. The shift of onset temperature of exothermic reaction on differential scanning calorimetric behaviors of the Co-substituted Li[Ni0.5-xCo2xMn0.5-x]O-2 (x = 0.025 and x = 0.05) was observed. (c) 2007 The Electrochemical Society.