The 5-V lithium insertion materials of Li[Ni1/2Mn3/2]O-4 having different primary particle sizes were prepared by the two-step solid-state reaction at several temperatures and characterized by Fourier transform infrared, X-ray diffraction, and scanning electron microscopy. The primary particle size of Li[Ni1/2Mn3/2]O-4 depends on the heating temperature. High temperature synthesis gives highly crystallized Li[Ni1/2Mn3/2]O-4 having large particle sizes with smooth {111} facets of octahedra characteristic of spinel. The steady-state polarization measurements were carried out by applying the sinusoidal voltage of peak amplitude of 1 V at 0.1 Hz to the cell consisting of Li[Ni1/2Mn3/2]O-4 and Li[Ni1/2Mn3/2]O-4. The well-developed primary particles prepared at 1000 degrees C were superior to the small primary particles prepared at temperatures lower than that value in terms of polarization together with cycling stability. (C) 2011 The Electrochemical Society. [DOI:10.1149/1.3531965] All rights reserved.