We obtained a highly delithiated sample of Li0.04NiO2 using acid leaching. The Rietveld analysis of its X-ray diffraction pattern indicated that the sample was a mixture of three phases. Two of them maintained the pristine structure with cubic close-packing layers while the other phase had a cadmium iodide structure with hexagonal close-packing layers. We examined the electrochemical behavior of this sample as a positive electrode material for lithium batteries. The discharge capacity was smaller than that of a Li0.10NiO2 sample obtained by acid leaching, suggesting that the capacity loss is due to the formation of the phase with the cadmium iodide structure.