Degradation mechanisms in doped spinels of LiM0.05Mn1.95O4 (M = Li, B, Al,Co, and Ni) for Li secondary batteries

Spinel lithium manganese oxides with a nominal composition of LiM0.05Mn1.95 O4 (M = Mn, Li, Al, Co, Ni, or B) are prepared and their degradation mechan isms encountered in lithium secondary cells are investigated. Among the deg radation mechanisms proposed in previous reports, those arising either from cation mixing or from the formation of oxygen-deficient spinels are neglig ible in these materials, but a certain amount of spinel dissolution is obse rved. X-ray diffraction (XRD) analysis indicates that the spinel lattice ex periences 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+ insertio n/removal is more significant in the B-doped spinel, but it is smaller in t he case of Ni-, Co-, Al-, or Li-doped spinels. Spinels experiencing a small er volume change maintain their structural integrity, even after prolonged cell cycling, such that there is a better capacity retention. In the B-dope d spinel, however, the spinel lattice is largely collapsed and new phases a re formed after cell cycling. This results in poor cycleability. It is prop osed that the structural breakdown due to the repeated change in lattice vo lume is the most important failure mode in these materials. Spinel dissolut ion plays a second major role. (C) 2000 Elsevier Science S.A. All rights re served.