High-voltage spinel LiNi_0.5Mn_1.5O₄ (LNMO) were synthesized by a spray pyrolysis method followed by heat treatment at 900^oC for 8 hours then post annealing at 700^oC for varies duration (0-96 hours) under air atmosphere. The crystalline structure and morphology of the prepared samples are studied with X-ray diffraction (XRD), neutron powder diffraction (NPD), Raman spectroscopy, scanning electron microscope (SEM), and high-resolution transmission microscope (HR-TEM). The electrochemical properties of the prepared materials had also been investigated by the studies of cyclic voltammetry (CV) and the capacity retention with coin-type cells by use of battery testing system. The results show all of the prepared samples have similar morphology and average particle size, but crystalize into mixtures of ordered P4₃32 and disordered Fd-3m phases with various fractions. It can be found from Table 1 that the phase fraction of ordered phase obtained from the Rietveld refinement of NPD patterns increases with increasing duration of post annealing at 700^oC. Fig. 1 manifests the results of cycling performance of the coin cells comprised with the prepared samples at 0.1 and 1.0 C rates, respectively. Samples containing the higher phase fraction of disordered phase exhibits the lower capacity fade than those dominated with ordered phase. It may be attributed to the ordered phase dominated sample experienced more complex phase change than the disordered phase dominant sample, which had been revealed from the in-situ NPD studies.