Different series of LiMn2O4-based spinels were studied, all presenting two
reduction steps at 4.5 and 3.3 V in addition to the "normal" spinel plateau
s around 4 V. A correlation was found between the capacity recovered on the
se additional steps, the manganese oxidation degree, and the cell parameter
of a given spinel. By means of in situ synchrotron diffraction we were abl
e to detect the appearance of a new set of diffraction peaks upon oxidation
of the 3.3 V step at 3.95 V, that disappeared on subsequent reduction at 3
.3 V. Electron diffraction and high resolution electron microscopy studies
on partially delithiated samples revealed the formation of double hexagonal
layers upon oxidation, consistent with the additional peaks observed in th
e in situ experiments. Finally, a model to explain the existence of the red
ox steps at 4.5 and 3.95/3.3 V based on the creation of these double hexago
nal layers is proposed. (C) 2000 The Electrochemical Society. S0013-4651(99
)07-060-3. All rights reserved.