Conventionally neglected mechanism of reversible redox reactions by oxygen ions in lithium-nickel oxide materials (LNO; Li2x Ni2-2x O2, 0 < x < 1) is proposed as a primary cause of unexpectedly high energy density of Li-ion battery. Using first-principles density functional theory calculations, cluster expansion theory, and Monte Carlo simulations, we unveil the underlying mechanism that is ascribed to the phase transition between layered and rocksalt structures initiated by cation disordering of Li and Ni at certain Li composition. At x = 0.5, the oxygen ions are put under specific chemical bondings of straight-linear type LiOLi configuration. They enable an active oxygen redox reaction involving peroxo (O2 2−) and superoxo (O2 −) ions, shown to dramatically increase the energy density of the LNO cathode. Using Monte Carlo simulations, we identify the proportional information to find the LiOLi configurations around the synthetic temperature of LNO materials. Our results indicate that the cation-disorder is a driving force for the oxygen redox via formation of the specific bondings. On the basis of our study, it is expected to provide useful guidelines for the design of Li-ion batteries with high energy densities beyond conventional ones.