Guided by in situ synchrotron X-ray diffraction (SXRD) measurements and quantitative analysis, we identified and synthesized a new type of lithium nickel oxide as a cathode material for Li-ion batteries. This compound is a composition of two crystallographic components: a major stoichiometric layer-structured phase, LiNiO2, and a minor Li-containing disordered rock-salt phase, LiδNi1-δO, both embedded in the same cubic close-packed oxygen lattice. The LiNiO2|LiδNi1-δO composite, when used as the active material in a cathode, delivers high energy density, superior rate capability, and long-term cycling stability owing to its unique structural configuration.