Well-crystallized nonstoichiometric LiNi0.5Mn1.5O4-delta thin-film electrodes were prepared by pulsed laser deposition (PLD). The charge/discharge behavior of the nonstoichiometric LiNi0.5Mn1.5O4-delta thin film is consistent with the film having a disordered spinel structure with space group Fd (3) over barm. These thin-film electrodes exhibit excellent cycle performance and rate capability. The measured Li diffusivity in the film is in the range of 10(-12) to 10(-10) cm(2)/s, which is comparable to that of layered LiCoO2. First principles calculations on the stoichiometric (ordered) spinel LiNi0.5Mn1.5O4 find no intermediate ordered phases between LiNi0.5Mn1.5O4 and Li0Ni0.5Mn1.5O4, leading us to speculate that voltage steps observed in experiments are caused by Ni-Mn disorder, which facilitates Li/vacancy ordering. The presence of a smaller step in "ordered" LiNi0.5Mn1.5O4 than in disordered LiNi0.5Mn1.5O4 is consistent with the calculation and may indicate presence of partial disorder even in ordered material. Although a maximum was found for the chemical diffusion coefficient of Li in LixNi0.5Mn1.5O4-delta near x=0.5, the self-diffusion coefficient exhibits a minimum at that composition. (c) 2007 The Electrochemical Society.