Conventional X-ray powder diffraction analyses of aluminum-substituted lithium nickel oxides showed pronounced broadening of the (11<i>l</i>) type peaks with increasing aluminum concentrations. It was postulated that a segregation tendency of nickel and aluminum in the layered lithium nickel oxide structure could lead to anisotropic strains and size effects for the (110) type planes and thus pronounced broadening. Variation in the distribution of aluminum and nickel was detected among different crystals by energy-dispersive X-ray spectroscopy (EDX) and within individual crystals at the nanometer-scale by electron energy loss spectroscopy (EELS). Synchrotron X-ray powder diffraction analyses of the “LiNi_1−<i>y</i>Al_<i>y</i>O₂” (0.10 ≤ <i>y</i> ≤ 0.50) samples revealed that strains in the (110) planes continuously increased with the aluminum concentration, which was then confirmed by convergent beam and selected area electron diffraction studies. Therefore, a combination of synchrotron X-ray powder diffraction, electron diffraction, EDX, and EELS analyses provided, for the first time, direct evidence for the segregation tendency of nickel and aluminum in the layered lithium nickel oxide structure, from which a domain microstructure for the “LiNi_0.50Al_0.50O₂” sample was proposed.Read less <