Spinel LiNi0.5Mn1.5O4 powders with different space group of Fd (3) over barm and P4(3)32 were synthesized by an ultrasonic spray pyrolysis method. The morphologies of the prepared powders depended on their structures; polyhedral shape for Fd (3) over barm and octahedral for P4332. The obtained discharge capacities of the Fd (3) over barm and P43(3)2 were about 60 mAh g(-1) and 70 mAh g(-1), respectively. Structural changes of both compounds were investigated on 3 V region by ex situ X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). From the ex situ XRD and HRTEM studies, LiNi0.5Mn1.5O4 having P4(3)32 symmetry showed that structural transition occurred reversibly from simple cubic (P4(3)32) to tetragonal phase (I4(1) / amd), whereas irreversible structural changes were observed for Fd (3) over barm symmetry on 3V region. (C) 2005 The Electrochemical Society.