The understanding of reaction mechanisms of electrode materials is of significant importance for the development of advanced batteries. The LiMn2O4 cathode has a voltage plateau around 2.8 V (vs. Li+/Li), which can provide an additional capacity for Li storage, but it suffers from a severe capacity degradation. In this study, operando X-ray diffraction is carried out to investigate the structural evolutions and degradation mechanisms of LiMn2O4 in different voltage ranges. In the range of 3.0–4.3 V (vs. Li+/Li), the LiMn2O4 cathode exhibits a low capacity but good cycling stability with cycles up to 100 cycles and the charge/discharge processes are associated with the reversible extraction/insertion of Li+ from/into Li x Mn2O4 (0 ≤ x ≤ 1). In the range of 1.4–4.4 V (vs. Li+/Li), a capacity higher than 200 mAh/g is achieved, but it rapidly decays during the cycling. The voltage plateau around 2.8 V (vs. Li+/Li) is related to the transformation of the cubic LiMn2O4 phase to the tetragonal Li2Mn2O4 phase, which leads to the formation of cracks as well as the performance degradation.