Spinels with nominal composition Li1.02Al0.25Mn1.75O3.97S0.03, Li1.02Al0.25Mn1.75O4, and Li1.02Al0.15Mn1.85O3.96S0.04 have been evaluated for their suitability as positive electrode materials in rechargeable lithium-ion batteries for electric (EV) and hybrid electric vehicle (HEV) applications. Li-7 magic angle spinning, nuclear magnetic resonance, X-ray diffraction, (XRD) and energy-dispersive spectroscopy experiments indicate that sulfur is most likely present as a trace impurity on the surface of the spinel particles rather than substituting for oxygen ions in the bulk, so it is unlikely to account for the previously reported enhanced cyclability of this material. Rather, the unusual particle morphology produced during calcination of some samples in the presence of sulfur compounds appears to impede (but does not completely prevent) conversion to the tetragonal phase that occurs at 3 V vs. Li and ameliorates the capacity fading associated with it. These materials exhibit reduced rate capability and capacity at 4 V, making them unsuitable for high energy density EV, or high power density applications. (C) 2003 The Electrochemical Society.