Layered Li1+y Ni x Co1−2x Mn x O2 materials were prepared by a solid-state reaction at 950°C for 10h in air and characterized by various methods. The cell performance depended on the composition. The rate performance deteriorated as Co content (1−2x) decreased. On the other hand, it improved as excess Li content (y) increased. The change of cell performance associated with composition variations was closely related to both the structural disorder (cation mixing) and the electrical conductivity (volume resistivity). The degree of cation mixing depended greatly on Co content (1−2x). On the other hand, the volume resistivity depended greatly on excess Li content (y). In other words, the cell performance was improved by the reduction of cation mixing and volume resistivity. Only Ni K-edge X-ray absorption near edge structure (XANES) spectra showed edge shifts to higher energy due to the oxidation of Ni as either Co content (1−2x) or excess Li content (y) increased. The unit cell size decreased as either Co content (1−2x) or excess Li content (y) increased. They are probably caused by the change from Ni2+ ( r Ni = 0.69 Å) to the smaller Ni3+ ( r N i 3+ = 0.56 Å). From these results, it is assumed that the reduction of volume resistibity is closely related to the increase of Ni3+ concentration in Li1+y Ni x Co1−2x Mn x O2.