Spinel lithium manganese oxides with a nominal composition of LiM0.05Mn1.95O4 (M = Mn, Li, Al, Co, Ni, or B) are prepared and their degradation mechanisms encountered in lithium secondary cells are investigated. Among the degradation mechanisms proposed in previous reports, those arising either from cation mixing or from the formation of oxygen-deficient spinels are negligible in these materials, but a certain amount of spinel dissolution is observed. X-ray diffraction (XRD) analysis indicates that the spinel lattice experiences an appreciable change in volume during charge-discharge cycling. The extent of this change depends on the nature of dopant. Compared to the undoped spinel, the lattice expansion/contraction according to Li+ insertion/removal is more significant in the B-doped spinel, but it is smaller in the case of Ni-, Co-, Al-, or Li-doped spinels. Spinels experiencing a smaller volume change maintain their structural integrity, even after prolonged cell cycling, such that there is a better capacity retention. In the B-doped spinel, however, the spinel lattice is largely collapsed and new phases are formed after cell cycling. This results in poor cycleability. It is proposed that the structural breakdown due to the repeated change in lattice volume is the most important failure mode in these materials. Spinel dissolution plays a second major role.