The nominal LiMn2O4 and Li-doped spinels with different oxygen stoichiometry were prepared and investigated for capacity fading upon cycling at elevated temperatures. The discharge plateau at 3.2V originating from oxygen defects in manganese spinels is observed to grow very quickly to nearly a maximum scale in initial 15 cycles at 60°C. Meanwhile, the majority of capacity fading is lost. Therefore, the quick capacity fading in the initial stage is associated with the increase of oxygen deficiencies or oxygen loss upon cycling. It is proposed that the oxygen loss is originated from the decomposition of instable spinel phases that containing little Li cations on the 8a sites ([□1]8a[Mn2−x ]16d[O4−δ □ δ ]32e, etc.), which are formed upon charging to the upper voltage limit. This phenomenon is much severe for nominal LiMn2O4 spinels with oxygen deficiencies. After partial substitution of Mn with Li, part of the Li cations on the 8a sites will be retained upon charging to the upper voltage limit. Thereafter, the cycling performance can be improved for the stabilized spinel phases formed upon charging.