LiNi1−y Al y O2 (0.10≤y≤0.50) compounds have been synthesized by a coprecipitation method. The characterization of the samples by X-ray and neutron diffraction, associated with Rietveld refinement analysis, has shown that for all materials, about 5% extra-nickel ions are present in the interslab space. Charge−discharge cycling of LiNi1−y Al y O2 as positive electrode material in lithium cells has shown that aluminum substitution suppresses all the phase transitions observed for the LiNiO2 system. Good cycling stability was observed, but the capacity decreases from 125 to 100mAh/g by increasing the aluminum amount from 10 to 25% (3–4.15V range; C/20 rate).