Neutron diffraction is a unique technique to study Li-ion batteries because of its high sensitivity toward detecting lithium ions and ability to differentiate between different cations. This information is essential for understanding the subtle structure/property relationships of active electrode materials. In this study, neutron diffraction was utilized to probe the cation disorder in LiNiO2 with and without Al3+ substitution by different synthesis processes. The powder neutron diffraction revealed that a strong oxidizer rather than Al3+-doping greatly reduces Li+ and Ni2+ mixing. The amount of Ni2+ at Li-site is 6% for Li1-x(Ni0.75Al0.25)(1+x)O-2 synthesized from excess LiOH, while it is only 0.6% for Li1-x(Ni0.75Al0.25)(1+x)O-2 which was synthesized from excess Li2O2. The reduction of Ni2+ at Li-sites greatly improves electrochemical performance. The substitution of Al3+ stabilizes the hexagonal lattice of Li1-xNi1+x-yAlyO2 even for highly lithium-deficient phases. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.005207jes] All rights reserved.