A series of cathode spinel materials with the nominal composites Li1+x Mn2−x O4−z have been studied with respect to capacity fading at room temperature upon cycling, both before and after elevated temperature storage. It was found that capacity fading is closely related to oxygen defect degree. The greater the oxygen deficiency, the poorer the cycling performance. Two major factors exist controlling the oxygen deficiency. One factor is oxygen loss at high synthesis temperature or upon cycling to high upper voltage, which increases the oxygen defect degree in samples. The other is Mn dissolution into the electrolytes accompanied by decrease in the oxygen deficiency. The latter one is supported by low temperature DSC and XRD data indicating that the temperature-dependent phase transition from cubic to lower symmetry becomes slight for oxygen defect samples after 60°C storage.