The crystal chemistry and electrochemical performance of the layered LiNi0.5−y Co0.5−y Mn2y O2 and LiCo0.5−y Mn0.5−y Ni2y O2 oxide cathodes for 0≤2y ≤1 have been investigated. Li2MnO3 impurity phase is observed for Mn-rich compositions with 2y >0.6 in LiNi0.5−y Co0.5−y Mn2y O2 and 2y <0.2 in LiCo0.5−y Mn0.5−y Ni2y O2. Additionally, the Ni-rich compositions encounter a volatilization of lithium at the high synthesis temperature of 900°C. Compositions around 2y =0.33 are found to be optimum with respect to maximizing the capacity values and retention. The rate capabilities are found to bear a strong relationship to the cation disorder in the layered lattice. Moreover, the evolution of the X-ray diffraction patterns on chemically extracting lithium has revealed the presence of Li2MnO3 phase in addition to the layered phase for the composition LiNi0.25Co0.25Mn0.5O2 with an oxidation state of manganese close to 4+, which results in a large anodic peak at around 4.5V due to the extraction of both lithium and oxygen.