It is known that stoichiometric spinel, LiMn2O4, when used as a lithium-ion battery cathode? exhibits significant capacity fade on cycling at room temperature, whereas by making the spinel slightly nonstoichiometric the capacity retention on cycling is improved to a great extent. To help understand this difference in performance, X-ray diffraction (XRD) was used to investigate the spinel structure during lithium extraction and reinsertion. The stoichiometric spinel shows degradation during the first lithium extraction which becomes more severe at high rates. The XRD pattern measured at the end of 50 cycles showed significant loss of structural integrity, with several prominent peaks that were not present prior to cycling. In contrast, the nonstoichiometric spinel showed no evidence of degradation up to at least 300 cycles, even at high rates. It is suggested that in the case of stoichiometric spinel, lambda- MnO2, which forms upon extraction of lithium, accumulates during cycling and, because it becomes disconnected from the active electrode, capacity fade is significant.