Li-rich Li₂MnO₃-LiMO₂ solid solution positive electrode materials are expected to insist high capacity and have been actively studied in recent years. In this paper, we will review various characteristics of Li_(1+x)(Ni_0.15Mn_0.8Co_0.05) _(1-x)O₂. The sample showed a discharge capacity of 300 mAh / g or more on the initial charge at 4.8 V. After that, it showed stable charge / discharge characteristics at 4.6V. As a result of XRD analysis, a peak derived from the (-Mn-O or Li-Mn-) n honeycomb structure was observed near 20 °(2θ). When the molar content of Li was measured by ICP, it was found that Li was contained at 1.67 with versus to the transition metal, suggesting that a solid solution was formed.