LiNi0.5Mn0.5O2 samples were prepared from NiMnO3 and Li2CO3 in a range of temperatures from 900 to 1050 degrees C. Synchrotron X-ray diffraction analysis combined with X-ray absorption spectroscopy showed that LiNi0.5Mn0.5O2 segregated into one major Ni2+O-enriched phase and one minor Li2Mn4+O3-enriched phase, where the extent of segregation decreased with increasing synthesis temperature from 900 to 1050 degrees C. Scanning and transmission electron microscopy combined with energy dispersive X-ray spectroscopy revealed that the segregated domains exist in individual particles. Although all of the LiNi0.5Mn0.5O2 samples showed comparable specific capacity (similar to 200 mAh/g) and capacity retention at low current densities, the rate capability of LiNi0.5Mn0.5O2 of 900 degrees C is lower than that of LiNi0.5Mn0.5O2 of 1000 degrees C. As X-ray photoelectron spectroscopy analysis showed that all of the LiNi0.5Mn0.5O2 samples had comparable surface chemistry, the higher rate capability of LiNi0.5Mn0.5O2 of 1000 degrees C can be attributed to reduced cation segregation of Ni2+O-enriched domains in the layered structure of the major phase, having potentially faster lithium diffusion than that of LiNi0.5Mn0.5O2 of 900 degrees C. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3526309] All rights reserved.